I can reproduce this with Safari and Firefox as well. You need to click on one photo (first), then second, and if that doesn't appear, go back to the first (you need to try sometimes several times until it occurs).
The common thing I've noticed is that it happens after uploading an image to same thread and then reviewing it (or something at that sort. It's not consistent, but as you can see, it happens across all major browsers). It may even be Mac specific issue, I'm not sure.
Change to 720p:
Edit: I'm able to reproduce this after several times without uploading any photos. It just comes and goes (basically you should NOT to cache the images, and then it occurs. Once cached, there is no way to reproduce it (at least not easily).