Not hard to do at all, but I can't name any particular editors. I'd personally hack away at the ROM to find the addresses of the sprite data, or write a quick program to render the entire ROM to the screen at different offsets until you can see the sprites and work out where they're stored. You could then mess with the data using a hex editor - if you're feeling extra special, you could edit the checksum so that remains valid.
I've decided what I've said wasn't very helpful if you aren't into ROM hacking. Try looking on zophar.net to see if there are any suitable programs.
A few unavoidable limitations of sprite editing - you can never replace a sprite with a bigger sprite. Also, the collision detection is likely to be a little messed up if you change the shape of a sprite.