It's not in a sig file or photo or all of us would be getting the same alert when we viewed the same thread and page. I do not get the malware alert when I've gone to the pages people have been reporting. However, my security software blocks access to picturesof.net so I know it's working.
I suspect it was in a Google Ad. However, the way ads are delivered by Google is by your cookies and surfing. I can see a completely different series of ads than someone else. The only way to know which ad it was is to take a screenshot of your monitor, not just the malware warning, so I can see what ad is displaying on your monitor at the time of the alert. I can then block that ad and contact Google. Google does a very good job of identifying bad ads and removing so I suspect if it was a Google ad, it is gone now. Clear your cache and history, reboot and there should be no more problems.
picturesof.net has been blocked by me today.