diff --git a/.gitea/workflows/macos.yml b/.gitea/workflows/macos.yml
index db350b9..029a94a 100644
--- a/.gitea/workflows/macos.yml
+++ b/.gitea/workflows/macos.yml
@@ -35,7 +35,8 @@ jobs:
       # The job still succeeds quickly so `package`'s `needs:` stays satisfied.
       # Homebrew's xcodegen formula requires Xcode 15.3, which cannot be
       # installed on macOS 12 (#109). The script installs a pinned
-      # prebuilt release instead.
+      # prebuilt release instead. dmgbuild is not installed here — the
+      # test job does not package (#95).
       - name: Ensure host tools
         if: "!(startsWith(github.ref, 'refs/tags/') && contains(github.ref_name, 'prerelease'))"
         run: scripts/ensure-host-tools.sh
@@ -193,8 +194,10 @@ jobs:
 
       # scripts/package-release.sh runs `xcodegen generate` and dmgbuild;
       # see build-and-test for why brew is not used on macOS 12 (#109).
+      # INSTALL_DMGBUILD isolates dmgbuild in build/.venv-dmgbuild so
+      # the test job never pip-installs it (#95).
       - name: Ensure host tools
-        run: scripts/ensure-host-tools.sh
+        run: INSTALL_DMGBUILD=1 scripts/ensure-host-tools.sh
 
       - name: Package release
         run: scripts/package-release.sh
diff --git a/docs/23-assets.md b/docs/23-assets.md
index fe336b3..fcc257e 100644
--- a/docs/23-assets.md
+++ b/docs/23-assets.md
@@ -25,7 +25,7 @@ Cone-only mark for window/taskbar. Raster set:
 
 | File | Use |
 |------|-----|
-| `icons/dmg-background.png` (+ `@2x`, `.svg`) | macOS DMG window (ice cream / wordmark scene). Headless `dmgbuild` after #189 |
+| `Resources/dmg-background.png` (+ `@2x`; source `brand/dmg-background.svg`) | macOS DMG window. `scripts/package-release.sh` builds a HiDPI TIFF and passes it to `dmgbuild==1.6.7` from `build/.venv-dmgbuild` (created by `INSTALL_DMGBUILD=1 scripts/ensure-host-tools.sh`). Monterey Python 3.9 needs `PIP_IGNORE_REQUIRES_PYTHON=1` or pip will keep 1.6.5. Missing art is a hard fail (#95, #189). |
 | `icons/wix-banner.bmp`, `wix-dialog.bmp` | MSI |
 | `icons/nsis-header.bmp`, `nsis-sidebar.bmp` | NSIS |
 
diff --git a/project.yml b/project.yml
index 642e5c3..b706f78 100644
--- a/project.yml
+++ b/project.yml
@@ -21,6 +21,8 @@ targets:
           - ICCery.entitlements
           - ICCery.Debug.entitlements
           - Argyll
+          - dmg-background.png
+          - dmg-background@2x.png
       - path: Resources/Argyll
         type: folder
     dependencies:
diff --git a/scripts/dmgbuild-settings.py b/scripts/dmgbuild-settings.py
index be4e87c..bd72f2f 100755
--- a/scripts/dmgbuild-settings.py
+++ b/scripts/dmgbuild-settings.py
@@ -1,10 +1,10 @@
 #!/usr/bin/env python3
 # scripts/dmgbuild-settings.py
 #
-# dmgbuild settings for ICCery. Set DMG_APP, DMG_FILENAME and DMG_VOLUME_NAME
-# in the environment, or accept the defaults. Background art can be supplied
-# later by placing a PNG at Resources/dmg-background.png and setting
-# DMG_BACKGROUND.
+# dmgbuild settings for ICCery. scripts/package-release.sh exports
+# DMG_APP, DMG_FILENAME, DMG_VOLUME_NAME, and DMG_BACKGROUND (a
+# HiDPI TIFF). A missing background is a hard error — a grey
+# Finder window is not an acceptable release artefact (#95).
 
 import os
 import sys
@@ -23,15 +23,23 @@ if not app_path or not app_path.endswith('.app') or not os.path.isdir(app_path):
 
 files = [app_path]
 
-# Background art is optional. If the referenced PNG does not exist, fall back
-# to a plain window. See docs/23-assets.md for the DMG background spec.
-background = os.environ.get('DMG_BACKGROUND', 'Resources/dmg-background.png')
-if background and not os.path.exists(background):
-    background = None
+# Finder on Sonoma+ is picky about PNG-with-alpha window pictures and
+# about classic Alias Manager blobs. package-release.sh always passes
+# a flattened HiDPI TIFF as DMG_BACKGROUND. dmgbuild 1.6.7 (bookmark
+# .DS_Store) needs Python >= 3.10, which the Monterey runner does not
+# have; 1.6.5 + TIFF is what CI can ship (#95).
+background = os.environ.get('DMG_BACKGROUND', '')
+if not background or not os.path.isfile(background):
+    sys.stderr.write(
+        'error: DMG_BACKGROUND must point at an existing image '
+        '(got %r)\n' % background)
+    sys.exit(1)
 
 icon = None
 
 # Window size is enough for the app icon and the Applications alias.
+# Bitmap is slightly larger than this rect so title-bar chrome on
+# 14+ does not crop the wordmark.
 window_rect = ((100, 100), (660, 400))
 
 # Use icon view without extra chrome.
diff --git a/scripts/ensure-host-tools.sh b/scripts/ensure-host-tools.sh
index 9c0afef..5924edc 100755
--- a/scripts/ensure-host-tools.sh
+++ b/scripts/ensure-host-tools.sh
@@ -2,19 +2,79 @@
 # scripts/ensure-host-tools.sh
 #
 # Bootstrap host tools needed by CI on the macOS 12 runner:
-#   - xcodegen: pinned prebuilt release from GitHub (Homebrew's current
-#     formula requires Xcode 15.3, which cannot be installed on macOS 12).
-#   - dmgbuild: via pip3 (used by scripts/package-release.sh).
+#   - xcodegen: always. Pinned prebuilt release from GitHub (Homebrew's
+#     current formula requires Xcode 15.3, which cannot be installed on
+#     macOS 12).
+#   - dmgbuild: only when INSTALL_DMGBUILD=1 or --dmgbuild. Isolated in
+#     build/.venv-dmgbuild so the test job never pip-installs it.
 #
-# Safe to run repeatedly: existing tools are left alone.
+# dmgbuild 1.6.6+, ds_store 1.3.2+ and mac_alias 2.2.3 declare
+# Requires-Python >= 3.10. The wheels are py3-none-any and run on the
+# runner's 3.9; PIP_IGNORE_REQUIRES_PYTHON is required or pip will only
+# offer 1.6.5 and keep a cached venv on that version (#95).
+# pip itself is capped at <26.1: 26.1+ needs Python 3.10.
+#
+# Safe to run repeatedly: existing tools are left alone unless the
+# dmgbuild pin is not met.
 
 set -eu
 
+ROOT="$(CDPATH='' cd -- "$(dirname -- "$0")/.." && pwd)"
 XCODEGEN_VERSION="2.38.0"
 INSTALL_ROOT="${XCODEGEN_HOME:-$HOME/.local/xcodegen/$XCODEGEN_VERSION}"
+VENV="$ROOT/build/.venv-dmgbuild"
+DMGBUILD_PIN="1.6.7"
 
-echo "==> Ensuring dmgbuild"
-python3 -c "import dmgbuild" 2>/dev/null || pip3 install dmgbuild
+INSTALL_DMGBUILD="${INSTALL_DMGBUILD:-0}"
+for arg in "$@"; do
+    case "$arg" in
+        --dmgbuild) INSTALL_DMGBUILD=1 ;;
+    esac
+done
+
+if [ "$INSTALL_DMGBUILD" = "1" ]; then
+    echo "==> Ensuring dmgbuild==$DMGBUILD_PIN in $VENV"
+    mkdir -p "$ROOT/build"
+    # pip 26.1+ requires Python 3.10 (dataclass slots). A leftover
+    # `pip install --upgrade pip` on this 3.9 venv installed 26.2.1 and
+    # the next pip invocation crashed. Recreate if pip is already dead.
+    if [ -x "$VENV/bin/python" ] \
+        && ! "$VENV/bin/python" -m pip --version >/dev/null 2>&1; then
+        echo "==> venv pip is broken; recreating $VENV"
+        rm -rf "$VENV"
+    fi
+    if [ ! -x "$VENV/bin/python" ]; then
+        python3 -m venv "$VENV"
+    fi
+    # Without this, pip on Python 3.9 hides 1.6.6+ and leaves 1.6.5.
+    PIP_IGNORE_REQUIRES_PYTHON=1
+    export PIP_IGNORE_REQUIRES_PYTHON
+    "$VENV/bin/python" -m pip install --upgrade 'pip>=24.3,<26.1'
+    "$VENV/bin/python" -m pip install --upgrade --force-reinstall \
+        "dmgbuild==$DMGBUILD_PIN" \
+        'ds_store>=1.3.3' \
+        'mac_alias>=2.2.3'
+    "$VENV/bin/python" -c 'from importlib.metadata import version
+print("dmgbuild", version("dmgbuild"))
+print("ds_store", version("ds_store"))
+print("mac_alias", version("mac_alias"))
+parts=[]
+for p in version("dmgbuild").split("."):
+    try:
+        parts.append(int("".join(c for c in p if c.isdigit()) or "0"))
+    except ValueError:
+        parts.append(0)
+parts += [0, 0, 0]
+raise SystemExit(0 if tuple(parts[:3]) >= (1, 6, 7) else 1)
+'
+    if [ -n "${GITHUB_PATH:-}" ]; then
+        echo "$VENV/bin" >> "$GITHUB_PATH"
+    fi
+    PATH="$VENV/bin:$PATH"
+    export PATH
+else
+    echo "==> Skipping dmgbuild (set INSTALL_DMGBUILD=1 for the package job)"
+fi
 
 if command -v xcodegen >/dev/null 2>&1; then
     echo "==> xcodegen already on PATH: $(xcodegen --version)"
diff --git a/scripts/package-release.sh b/scripts/package-release.sh
index 88ef010..872688e 100755
--- a/scripts/package-release.sh
+++ b/scripts/package-release.sh
@@ -103,19 +103,39 @@ EOF
     scripts/verify-sidecar-signatures.sh "$APP"
 fi
 
-echo "==> Installing / locating dmgbuild"
+echo "==> Locating dmgbuild"
+# The package CI job already ran INSTALL_DMGBUILD=1 ensure-host-tools.sh,
+# which created build/.venv-dmgbuild and prepended it to PATH. Local
+# runs bootstrap the same venv if dmgbuild is missing.
+VENV="$ROOT/build/.venv-dmgbuild"
 if ! command -v dmgbuild >/dev/null 2>&1; then
-    VENV="$ROOT/build/.venv-dmgbuild"
-    if [ ! -d "$VENV/bin" ]; then
-        python3 -m venv "$VENV"
-        "$VENV/bin/pip" install --upgrade pip
-        "$VENV/bin/pip" install dmgbuild
+    if [ ! -x "$VENV/bin/dmgbuild" ]; then
+        INSTALL_DMGBUILD=1 "$ROOT/scripts/ensure-host-tools.sh" --dmgbuild
     fi
     PATH="$VENV/bin:$PATH"
     export PATH
 fi
 if ! command -v dmgbuild >/dev/null 2>&1; then
-    echo "error: dmgbuild not available. Try 'python3 -m venv .venv && pip install dmgbuild'" >&2
+    echo "error: dmgbuild not on PATH; run INSTALL_DMGBUILD=1 scripts/ensure-host-tools.sh" >&2
+    exit 1
+fi
+echo "dmgbuild $(command -v dmgbuild)"
+if [ -x "$VENV/bin/python" ]; then
+    "$VENV/bin/python" -c 'from importlib.metadata import version; print("dmgbuild", version("dmgbuild"))'
+fi
+
+PNG1X="$ROOT/Resources/dmg-background.png"
+PNG2X="$ROOT/Resources/dmg-background@2x.png"
+if [ ! -f "$PNG1X" ] || [ ! -f "$PNG2X" ]; then
+    echo "error: missing $PNG1X or $PNG2X" >&2
+    exit 1
+fi
+mkdir -p "$ROOT/build"
+DMG_BACKGROUND="$ROOT/build/dmg-background.tiff"
+echo "==> Building HiDPI DMG background TIFF"
+tiffutil -cathidpicheck "$PNG1X" "$PNG2X" -out "$DMG_BACKGROUND"
+if [ ! -f "$DMG_BACKGROUND" ]; then
+    echo "error: tiffutil did not write $DMG_BACKGROUND" >&2
     exit 1
 fi
 
@@ -128,6 +148,7 @@ VOLUME_NAME="ICCery ${VERSION}"
 DMG_APP="$APP" \
 DMG_FILENAME="$DMG" \
 DMG_VOLUME_NAME="$VOLUME_NAME" \
+DMG_BACKGROUND="$DMG_BACKGROUND" \
 dmgbuild -s scripts/dmgbuild-settings.py "$VOLUME_NAME" "$DMG"
 
 echo "DMG: $PWD/$DMG"