Different shapes, colors... what do you need exactly? I can do it easily for you, just clarify a little on exactly what you want.
If you want, check out my resources (link in my signature) to see some of my other contributions, namely CharacterSets(Map Elements), which are exactly what you want, though plants rather than rocks.
So, I'll do this, no problem, just need a bit more details.
ALSO!! What size do you want them to be? a single 32x32 ,a tall 32x64, or a large 64x64?