The images I am unable to delete aren't currently in use on the forum.

I did a test and went into an active forum and changed the image to one I can't delete. It did show on the board. So you're right they are somewhere, where is unknown.

The board is http://www.freaksforum.com/forum/ubbthreads.php/forum_summary

The images you see on the board are what I am talking about. However the images I can't delete are football logos.

For example the ones with the arrows are what I am looking to delete and there are others like them.

[Linked Image from img46.imageshack.us]

Not sure if any of this helps but hopefully so.