Actually there is a standard that most games use for storing graphics, since the SNES expects to find them in that format in VRAM anyway. If the graphics aren't compressed you should be able to edit them with any tile editor that supports the SNES format, a lot of the more popular games are pretty large and have to use compression to fit all their data though.
I have a tile viewer (Not Editor) I wrote a while ago that supports GB/GBC/GBA, and SNES graphics formats, I'll upload it at KG some time soon if I can find it, but it's incredibly slow partly cause it's written in Game Maker so you'd be better off finding a different one anyway.