f39-backgrounds-39.0.2

This release provides different versions of the f39 background images which "avoid color banding by bayer dithering and using 8bit rather than 16bit depth of the original", according to @jimmac (I have no idea what that means except 'it looks better'). It also drops an attempt to 'optimize' the images with pngquant at install time which just resulted in them looking worse.