you could use the selector tool to get the wall section. Then paste a beach image on there. Of course, depending on how good it was, there are probably effects he did with lighting to make it look like the sun was shining from the beach.
Also, if the wall is all one color, you could use the auto select and drag until you get the amount you want. Not too sure about preserving shadows though. It wouldn't be too hard to just copy them and set them to be a bit translucent though.
EDIT: I agree, Art section would get you more sure responses on how to do stuff. I personally don't do any image editing, so there are probably (like, i'm pretty darn sure) a lot of advanced techniques that would make it a WHOLE LOT easier.