ABC News' George Stephanopoulos Reports: When John McCain began his acceptance speech last night, the staging was decidedly less grand than the sweeping stadium vista that the Obama campaign constructed at Invesco Field in Denver last week. And purposefully so. It was more intimate, with the candidate standing closer to his supporters, with a giant screen behind him.A giant green screen. Or so it appeared to TV viewers for the first several minutes of the speech.McCain has had trouble with green screens before, when internet mischief-makers used a green backdrop at one of his worst-received speeches of the year in June to superimpose other images behind -– making for some popular YouTube spoof videos.So, had the McCain campaign set designers really picked an image for the screen that highlighted the same unfortunate shade of green?On close up the viewer only saw green, but on a wider shot, and to those in the convention center itself, the green was part of a lawn in front of a distinguished white building. What building?A little Googling reveals that the photo appears to be the exterior of Walter Reed Middle School in North Hollywood, California. Was this a school McCain attended or is otherwise significant to the campaign? Symbolic of his message, perhaps? The campaign isn't saying. But there is speculation on the web, asking, could it simply have been an error and Walter Reed Medical Center was the intended image?No comment at this writing from the McCain campaign.