VSNES can help you with this.
Play the game until the graphics you want to edit are on screen, then make a savestate and open that savestate in VSNES. Find the graphics in VRAM. If they match what you found in the ROM (ignoring the colors!), congratulations: you found them on the first try. Otherwise, the game is pulling some kind of trickery between the ROM and the screen. Find the graphics data in the RAM viewer - that is, the regular SNES memory viewer, not the VRAM. It should be in there, somewhere. Once you do, make note of the address and open up Geiger's Snes9x Tracer.
Start the game up, but before the graphics show up on screen, pause set a write breakpoint to the address where you saw them in RAM. This will catch the game in the act of writing data to that location. From there, you can work backwards: each byte has to have come from somewhere. It'll take a few iterations (and possibly a few do-overs if your timing is off), but you should be able to trace the routine all the way back to its source in the ROM, and somewhere in there will be the compression routine.
Just so you know, compression is easy to crack. The game provides its own blueprint for the decompressor. It's the recompression that can be a pain, but as long as you know how the original compression works, it shouldn't be too hard.