I think that must have been the problem. I discussed it with my colleague and we think I must have missed the guide and grabbed the photo underneath. When I didn't move the guide, I am sure I clicked again in an attempt to grab it, at which point the arrow turned into a hand to move the photo, and then when I clicked on the artboard, it changed back to the selection tool. I seem to be having a problem selecting guides in 6, though, which I didn't have in 4. I wish there was a way to adjust the tolerance of the tool.
Thanks for your help!