Ok, I decided to make a contribution to the much-loved homebrew scene, so here it is, my first program, PSPpaint! :)
It's still lacking lots of things I want it to have, but I guess it's already mature enough for release.
PSPpaint features a sizeable brush and a color palette (for now). You can also save the pictures (they get saved in ms0:/PSP/PHOTO/PSPpaint, so you can view them outside PSPpaint). Saving creates always a new file with filename picture_x.png where x is the picture number.
I've developed it on 2.71, but it should work on other versions via eLoader (Insomniac tested it on 1.5, works fine according to him) or other ways.
I just updated PSPpaint to version 0.51, fixing some minor annoyances (no new features yet, wait for 0.6 :)). I also made a new palette, which now includes the grayscales too :) The download links below are updated, but if you really need to get an older version, you can download them here.
KXploited version for FW version 1.5
Single EBOOT version for other FW versions