There's no easy way to do it (although Thorpey's suggestion of Search is pretty good).
You can also do it by finding a post by that user in the gallery, clicking on their username to view their profile, then clicking on "Photos" to see a list of all their photos.
I am considering doing a more complete integration between the forums and the gallery like I did over on ZooChat - it seems to work really well there with nearly 80,000 photos from zoos around the world! Only problem is that it takes a lot of effort to set up, so I need to find the time first - but I think it will be worthwhile once it is done.