I wonder if someone could have hijacked an image on photobucket and replaced it with something malicious - I've certainly heard of images being created that run javascript when called, and then the javascript can wreak havoc. The affected image could have also just been uploaded as a compromised file in the first place, most likely unbeknown to the uploader.
If there is a genuine pattern of the only threads being affected all including posts from one person, that probably suggests the 'bad' image is in their account
Irrespective of whether what I've speculated on is true or not, It's always worth bearing in mind that when you download an image from a website somewhere, all may not be as it seems.
If you have some form of image editing software it doesn't hurt to load it up and save it as a different file, it may remove any malicious entries from the image file, depending on the software used.
