I know that indexed PNG (further coptimized with the PNGOptimizer or PNGout) results in very small files.
Its the standard format for Mobile games.

From my tests, the PNGs worked good as Panel elements.

Some small recent games of mine:
Griefer 4k http://damocles11.byethost9.com/griefer4k.html
Kobold 4k http://damocles11.byethost9.com/kobold.html
Cave 4k http://damocles11.byethost9.com/cave4k.html