You are exactly right that it was the 4 pictures side by side that was the culprit.
It also happens when someone does a huge line of ellipses, or a run-on sentence without spaces. Contacting the moderator of that forum, or us if there isn't a mod, is the only solution. It only takes a second to edit the post and fix whatever is causing the problem.
I'm with you- scrolling to the side drives me batty. Thankfully, there are so few things that cause it to happen on ATP that we don't see it very often!
