What makes me believe it is that although it would be possible to fake it to that degree of realism. The person would need to spend alot of time and be an professional artist with years of experience in something like cinematic matte painting to do so. Like Craig Mullins for example.
The reason I say this is because of the correct shadow perspective, shadow tinting, specular highlighting etc..
I strongly doubt anyone of that level of skill would consider wasteing their time on a simple prank, and anyone who did want to waste their time of a prank like this wouldn't have the skill to pull it off to this degree of realism. So thats my hypothesis, I'll admit I havn't thought about it in terms of why is this photo is turning up out of the blue, but Techno gave a pretty reasonable explanation and frankly it doesn't bother me as to why. Just that it is.