Hi,
I've already done something exactly like this in Blender (LOVE that tool!). I placed an example in my portfolio, please have a look. You can have my blender file and accompanying image files.
The Blender file consists of a wall image with corresponding normal and displacement maps, plus a png file of some sample street art I found on google images.
Either image can be changed to whatever you need. If you want bricks, no problem. Or maybe you have your own version of street art you'd like to "paint" on the wall. Easy. Just let me know.
Mick
studioFUME