Sorta depends on what your after. Kimmo's 12 rounds thread has some nice characters done with Poser, Devin has drawn loads of characters and monsters presumably with a pen tablet, theres 3D rendered cars and sci-fi bits, real world clipped up textures for terrain. The dundjinni forums have loads of downloadable content for your app. Torstan is doing some nice dungeon bits, theres the RPTools forums, BattlegroundRPG forums, theres a big collection called the CUSAC which have thousands of items and bits of stuff. These are mostly PNG files with a transparency layer in them - the RGBA ones. You put the transparent bits with a low alpha (A) bit and the color into the RGB bit of it. Use photoshop or Gimp to do that. Dunno what else I can add really.