PokeDSPic is a tool that allows you to look at graphical assets such as Pokémon sprites and export and import PNGs to modify them. I'm not really sure where this tool came from but I know I used it to modify some sprites in HG/SS. There are two versions included in the download - one is for Diamond and Pearl only, and the other for PtHGSS only. With that said, the DP one is needed for some graphics in HG/SS such as trainer backsprites. (Thanks, Hiro!