I think it's because the WYSIWYG editor makes that image URL a link when typing up the reply, then surrounds the URL with the URL tag, and then that breaks it. Minor bug, but a bit annoying. Probably something vBulletin would have to fix, but I figured it was worth reporting, maybe you guys can report it to them.
[quote="yg17, post: 10046314"]Allow me to demonstrate, and apologies for the double post, but it's necessary to explain the bug.
timg tagged image of a cute kitty (although I bet this bug works with timg tagged image of cute puppies too):
[img]http://icanhascheezburger.files.wordpress.com/2008/06/funny-pictures-balogna-first-name-nom.jpg[/img][/QUOTE]