diff --git a/Buildscr b/Buildscr
index 2b789d4c..a98f5f3f 100644
--- a/Buildscr
+++ b/Buildscr
@@ -152,8 +152,7 @@ in putty do perl -i~ -pe 'y/\015//d;s/$$/\015/' LICENCE
 
 # Some gratuitous theming for the MSI installer UI.
 in putty/icons do make -j$(nproc)
-in putty do convert -size 164x312 'gradient:blue-white' -distort SRT -90 -swirl 180 \( -size 329x312 canvas:white \) +append \( icons/putty-48.png -geometry +28+24 \) -composite \( icons/pscp-48.png -geometry +88+96 \) -composite \( icons/puttygen-48.png -geometry +28+168 \) -composite \( icons/pageant-48.png -geometry +88+240 \) -composite windows/msidialog.bmp
-in putty do convert -size 493x58 canvas:white \( icons/putty-48.png -geometry +440+5 \) -composite windows/msibanner.bmp
+in putty do ./windows/make_install_images.sh
 
 mkdir putty/windows/build32
 mkdir putty/windows/build64
@@ -200,10 +199,20 @@ in putty/windows with wixonlinux do candle -arch x64 -dRealPlatform=x64 -dDllOk=
 in putty/windows with wixonlinux do candle -arch x64 -dRealPlatform=Arm -dDllOk=no -dBuilddir=abuild32/ -dWinver="$(Winver)" -dPuttytextver="$(Puttytextver)" installer.wxs && light -ext WixUIExtension -ext WixUtilExtension -sval installer.wixobj -o installera32.msi -spdb
 in putty/windows with wixonlinux do candle -arch x64 -dRealPlatform=Arm64 -dDllOk=no -dBuilddir=abuild64/ -dWinver="$(Winver)" -dPuttytextver="$(Puttytextver)" installer.wxs && light -ext WixUIExtension -ext WixUtilExtension -sval installer.wixobj -o installera64.msi -spdb
 
-# Bodge the platform fields for the Windows on Arm installers, since
-# WiX 3 doesn't understand Arm platform names itself.
-in putty/windows do ./msiplatform.py installera32.msi Arm
-in putty/windows do ./msiplatform.py installera64.msi Arm64
+# Change the width field for our dialog background image so that it
+# doesn't stretch across the whole dialog. (WiX's default one does; we
+# replace it with a narrow one so that the text to the right of it
+# shows up on system default background colour, meaning that
+# high-contrast mode doesn't make the text white on white. But that
+# means we also have to modify the width field, and there's nothing in
+# WiX's source syntax to make that happen.)
+#
+# Also bodge the platform fields for the Windows on Arm installers,
+# since WiX 3 doesn't understand Arm platform names itself.
+in putty/windows do ./msifixup.py installer32.msi --dialog-bmp-width=123
+in putty/windows do ./msifixup.py installer64.msi --dialog-bmp-width=123
+in putty/windows do ./msifixup.py installera32.msi --dialog-bmp-width=123 --platform=Arm
+in putty/windows do ./msifixup.py installera64.msi --dialog-bmp-width=123 --platform=Arm64
 
 # Sign the Windows installers.
 ifneq "$(cross_winsigncode)" "" in putty/windows do $(cross_winsigncode) -i https://www.chiark.greenend.org.uk/~sgtatham/putty/ -n "PuTTY Installer" installer32.msi installer64.msi installera32.msi installera64.msi
diff --git a/windows/make_install_images.sh b/windows/make_install_images.sh
new file mode 100755
index 00000000..49041e24
--- /dev/null
+++ b/windows/make_install_images.sh
@@ -0,0 +1,22 @@
+#!/bin/sh
+
+# Script to make the bitmap files that go into the PuTTY MSI installer.
+
+set -e
+
+# For convenience, allow this script to be run from the Windows
+# subdirectory as well as the top level of the source tree.
+if test -f installer.wxs -a ! -f putty.h -a -f ../putty.h; then
+  cd ..
+fi
+
+convert -size 164x312 'gradient:blue-white' -distort SRT -90 -swirl 180 \
+        \( icons/putty-48.png -geometry +28+24 \) -composite \
+        \( icons/pscp-48.png -geometry +88+96 \) -composite \
+        \( icons/puttygen-48.png -geometry +28+168 \) -composite \
+        \( icons/pageant-48.png -geometry +88+240 \) -composite \
+        windows/msidialog.bmp
+
+convert -size 493x58 canvas:white \
+        \( icons/putty-48.png -geometry +440+5 \) -composite \
+        windows/msibanner.bmp
diff --git a/windows/msiplatform.py b/windows/msifixup.py
similarity index 52%
rename from windows/msiplatform.py
rename to windows/msifixup.py
index eea4272f..4d4bb3e4 100755
--- a/windows/msiplatform.py
+++ b/windows/msifixup.py
@@ -16,30 +16,43 @@ def run(command, verbose):
         sys.stdout.write("".join(
             "> {}\n".format(line) for line in out.splitlines()))
 
-def set_platform(msi, platform, verbose):
-    run(["msidump", "-t", msi], verbose)
+def make_changes(msi, args):
+    run(["msidump", "-t", msi], args.verbose)
+    build_cmd = ["msibuild", msi]
 
-    summary_stream = "_SummaryInformation.idt"
+    def change_table(filename):
+        with open(filename) as fh:
+            lines = [line.rstrip("\r\n").split("\t")
+                     for line in iter(fh.readline, "")]
 
-    with open(summary_stream) as fh:
-        lines = [line.rstrip("\r\n").split("\t")
-                 for line in iter(fh.readline, "")]
+        for line in lines[3:]:
+            yield line
 
-    for line in lines[3:]:
-        if line[0] == "7":
-            line[1] = ";".join([platform] + line[1].split(";", 1)[1:])
+        with open(filename, "w") as fh:
+            for line in lines:
+                fh.write("\t".join(line) + "\r\n")
 
-    with open(summary_stream, "w") as fh:
-        for line in lines:
-            fh.write("\t".join(line) + "\r\n")
+        build_cmd.extend(["-i", filename])
 
-    run(["msibuild", msi, "-i", summary_stream], verbose)
+    if args.platform is not None:
+        for line in change_table("_SummaryInformation.idt"):
+            if line[0] == "7":
+                line[1] = ";".join([args.platform] + line[1].split(";", 1)[1:])
+
+    if args.dialog_bmp_width is not None:
+        for line in change_table("Control.idt"):
+            if line[9] == "WixUI_Bmp_Dialog":
+                line[5] = args.dialog_bmp_width
+
+    run(build_cmd, args.verbose)
 
 def main():
     parser = argparse.ArgumentParser(
         description='Change the platform field of an MSI installer package.')
     parser.add_argument("msi", help="MSI installer file.")
-    parser.add_argument("platform", help="New value for the platform field.")
+    parser.add_argument("--platform", help="Change the platform field.")
+    parser.add_argument("--dialog-bmp-width", help="Change the width field"
+                        " in all uses of WixUI_Bmp_Dialog.")
     parser.add_argument("-v", "--verbose", action="store_true",
                         help="Log what this script is doing.")
     parser.add_argument("-k", "--keep", action="store_true",
@@ -51,7 +64,7 @@ def main():
     try:
         tempdir = tempfile.mkdtemp(dir=msidir)
         os.chdir(tempdir)
-        set_platform(msi, args.platform, args.verbose)
+        make_changes(msi, args)
     finally:
         if args.keep:
             sys.stdout.write(