[Graphics] Having issues inserting new pokemon sprites to Pokemon Platinum.

I want to insert some slighly edited pokemon sprites of my own to Pokemon Platinum but for some reason PokeDSPic tells me the new file "has to be 8bpp indexed, otherwise it won't work". I've tried making new sprites with Paint, Paint Tool Sai 2 and GIMP and all of the tools create incompatible files that end up creating garbage when I insert them regardless so...Is there something I'm doing wrong? Or do I have to use a different tool altogether to create new sprites?