Re-/skinning your applications. Not for retards, just for readers edition.

Where to look:
    ur0:appmeta/GameID/
        - icon0.png (the skittle icon)
        - pic0.png (the image shown when launched, doesn't seem to work with PSP/1 (at first glance))
    ur0:appmeta/GameID/livearea/contents
        - bg0.png (the livearea background)
        - startup.png (the start gate icon)

These names might be different, and are based on defaults.
Refer to the livearea's layout information.

These seem to be deencrypted and cached by the system and left alone.
They also seem to survive reboots. A database rebuild might restore them.

 Item                | Resolution
----------------------------------
Live area background | 840x500
Skittles             | 128x128
Launch image (pic0)  | 960x544
Vita start gate icon | 280x158
PSP start gate icon  | 144x80 (original PSP)
PS1 start gate icon  | 80x80 (cover)

The live area background resolution can be used on any content type.
Despite being 80 pixels tall and 80/144 pixels wide, PSP and PS1 liveareas can use the Vita start gate icon resolution of 280x158.

All images must be PNG and use an indexed color palette!
When starting from scratch in GIMP go to [Image -> Mode -> Indexed] and use defaults when you're satisfied before the color conversion.