So, I did this tower [using GIMP]:

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I wanted to have a bit of detail in the structure of the walls, so, what I did is download a pattern (I believe it's called "stone-brick") and played with it to have it as a wall of two stone wide. Then I copy pasted it a few times to make my walls and used the cloning tool to fuse the various bits together relatively seamlessly.

I'm kinda happy with how that part turned out BUT, I have two questions:

First of all, is there a way to speed up the process? I tried saving the walls as a brush, but the product is far from seamless.
So, question 1, does anybody knows if there are any such brush floating around that you could put together to produce a seamless line, a bit like there are patterns that produce a seamless pattern even when repeated?
Is it even possible using a complex picture as a brush?

Also, and maybe more importantly, I am only somewhat happy with the result. Because the wall itself looks a bit rough. It would be great for some buildings, but I am trying to make a posh and sophisticated building (the marbled floor might have been a bit of a give away). I think it would fit the building's personality better if I had an actual brick or brick like structure: very homogeneous stones, maybe alternating, producing a very smooth finished wall.
Does anybody has such a wall structure lying around I could pilfer?