Installer/darwin: Use tiffutil for converting the images:
tiffutil -cathidpicheck MacOS/1240\ x\ 836.png MacOS/620\ x\ 418.png -out VirtualBox_mpkg/background.tif tiffutil -cathidpicheck 1160\ x\ 800.png 580\ x\ 400.png -out DiskImage/vbox_folder.tiff
bugref:10724