To update the DMG background image, create 1x and 2x versions of a background:

1x: background.png
2x: background@2x.png

Create a single TIFF combining both 1x and 2x background bitmaps:
Run 'tiffutil -cathidpicheck background.png background@2x.png -out background.tiff'

Use background.tiff as the DMG background
