OK, this is a bit of a puzzle yet, and explanations are entirely speculative so far, so I'm going with the one that seems fairly likely for the purposes of this post. Whatever the actual explanation, though, it can hardly be favorable to the McCain campaign, except inasmuch as everything is.
So, apparently, Candidate McCain chose to (or at least ended up) give his big acceptance speech, the highlight of the Republican Convention, in front of an image of a ritzy, dare-we-say elitist, North Hollywood middle school (not entirely dissimilar to an expensive McMansion, for that matter). Going with the reasoning of the Republicans and their pet pundits, that Obama's backdrop of Greek columns (much like George Bush's background when addressing the RNC, incidentally) was supposed to present him as a Greek god (so what was Bush going for? Roman emperor?), the only reasonable conclusion is that the McCain campaign chose a middle school backdrop to make McCain seem younger.
Kidding aside, it seems likely that it was a mixup due to the middle school sharing a namesake with Walter Reed Army Medical Center. I think it says something about the staffers of the McCain campaign that they would think it reasonable that Reed looks that beautifully kept up. It would seem to fit well with delusions that the Army's medical services are keeping things completely shipshape for our wounded veterans, not a speck of mold or anything, and no expense has been spared in keeping it lovely. If anything, it looks like we might have spent altogether too much on fancy architecture and groundskeeping! Except, of course, that it isn't. Just another notion based on fantasy.