first  : Only for FW 1.50 don't try it on other

i recommend to install some Custom Firmware ...

now edit pictures (you can use diffrent pallete, but only 256 colors!)

order? on screenshot.

For flashing you can use PSP filer, if you don't want use my.

Flasher:
Copy folders __SCE__tmprf and %__SCE__tmprf to GAME folder and run it
don't copy DATA folder, that is needed on PC

one thing, XMB shows images smaller (i think 60x64) but don't change resolution