The thing is with rock is that it's not made out of blobs, but rather out of strips of indentations in a topological order/way (almost like steps).
Here's an edit:
Another thing, the picture in the OP of Owlboy is upscaled, which makes it look bigger than it actually is, so you might want to consider making your picture upscaled or smaller (if you're going to make tiles it's going to be even more problematic in which case you'll definitely have to either make the picture smaller or make it upscaled).
