While it appears that your issue was the fact that the image was hosted on a flagged site, you may still want to consider updating your forum software as well as checking for strange logins from weird ips on top ranked forum accounts.

I once was too lazy to update my vbulletin forum, and somehow a version of the god_mode virus was injected along with a webshell. This somehow led to access to root admin accounts, and was followed with the uploading of numerous html pages meant to redirect the user to sites that would install malicious content while browsing the main site. My laziness lead to more effort than I had initially avoided. Being flagged as hazardous was the only immediate warning sign of the infection, and due to that, I was able to remedy the intrusion within 24 hours of occurance.  It only took one email to have my site's hazardous rating removed from Securi, and a rescan with the google webmaster tool that verified there was no threats to satisfy google. I do realize your situation is different, but it is wise to take precautions.

According to YaBB, The most recent version is 2.5.2, released in October 2012, as opposed to the 2.5 AE version you appear to be currently running which was released in July 2010. http://www.yabbforum.com/archives.php

Hey if you aren't gonna update do I have permission to use a flaw in this ancient version of YaBB to delete all the posts and replace them with donkey porn?