First off, I can see no reason snesgfx couldn't handle something like this. Unless the image is just too large.
Originally posted by Download page
This tool lets you rip images (png, bmp, jpg, etc) to SNES format with automatic color reduction using pngquant.
But it's more of a question of whether you should.
I'm actually rather surprised to find there isn't a general snesgfx tutorial thread in existence, here. Someone should make that.