Usenti

Usenti is a helpful utility for converting graphics for gba. It's basically MSPaint with some good palette tools and the ability to export to a gba format. Sometimes it's best to open an existing image in Photoshop/GIMP to resize it and save it as an indexed bitmap (256 or 16 color), then open it in usenti, make any necessary palette adjustments, then export to c files. Remember that the first entry in the palette (top left in the palette window) is transparent. You should check the "header" box when you export so usenti creates both a .h and a .c/.cpp file, which I haven't seen any other gba utilities do.