For the entire tileset, JCS only takes .bmp, .pcx, .lbm, and .anm files... I recommend .pcx because it's smaller than .bmp and yet something you can actually find programs that will save images as. It's up to you what sorts of files the parts of the tileset come from, but you don't want too many .jpg files as they use lots and lots of colors and you obviously have to observe JJ2 color limits.