Thanks for updating us, NLP-er. Much appreciated.
Sayid: thank you, that's extremely helpful. I believe I've pinpointed the problem; vBulletin has a setting to define the maxium height and width of even remotely-hosted signature images like that one. If an image is above these limits, vB treats it like a link, instead, hence the redirection.
I'll take a look at putting together a quick patch. Thanks again for the clarification.