Hi Retronio, I've done something similar in the past. I took some photos of buildings I made then printed them out using excel, this way I could overlap the image across several A4 sheets, as you can't get a good image printing up to the edge of the paper. I also found that when I print things out even if I set the image to exactly 20cm it never prints it that size.
(This caused me lots of problems when I first printed out my own wallpaper!)
If you use your own photo you can sent up some buildings or figures and it gives it a more 3d effect than if you just scan your walls.
I hope this helps if you need more info let me know.
