I've stumbled upon this a few times.
Yup, me too.
I figured it was because people were taking the BBCode that comes from TalkImg and then sometimes messing up the aspect ratio by fiddling with the image dimensions...
But, it looks like what's mostly going on is that having non-automatic dimensions is interacting poorly with this:
img.userimg { max-width: 100%; }
I don't know if this would be easy to improve.
I think it's going to be tricky to fix in a completely clean way.
I guess, a discussion-wise starting point might be:
img.userimg { max-width: 100%; height: auto; }
(As in, I think that that would fix the specific problem you're talking about, but, not without producing some other consequences, too.)
To everyone that's recently left me a post (or sent me a PM): I'm sorry that I've been so unresponsive lately. I'll get back into my usual rhythm soon.