I think I found the (previously unknown, apparently) source for the Shepard Fairey "Hope" poster. I stretched the
original a bit (really, a tiny amount) and flipped it horizontally,
but didn't need to rotate it at all.
A few minutes of Google image searching (for obama 2007) after reading this is all it took.
It looks like my match, while close enough to fool me & tons of other people, was actually the wrong image. A better match is linked to in stevesimula's comment, below.