You can create these rocks as a background or as doodads. I should choose for doodads because they are not big enough to really use as a background. So, take a look at the default doodads of Teeworlds. You will notice that these do not have a border, don't have much contrast, have neutral-bright colours and have shadows, rather than lightning.
These are just some things to pay attention at if you want a good result. I have to admit that these rocks look really tricky to make, and you need to do a lot of expirimenting to get right.
Also, take a look at google and search for the same rocks that you want to make. Maybe you come across some -images that will make you think: "Hey, that's actually what I want to make!", and you can use this as a good example.