Two plugins relating to your PSP and the screen you stare at: PRXshot v0.2.1 and Bright v0.3! By the names alone you can probably guess what each does: codestation’s PRXshot captures the screen while in-game or browsing the XMB by pressing the “Note” button; plum’s Bright lets you adjust the brightness / luminance levels on a totally different scale.

Both plugins are configurable by adjusting values in their respective ini files, and both should be compatible with the custom firmwares of today. If you want more info I suggest you check the source link(s).