Basically when you submit a message in the forums, you can upload up to two photos at a time.
Before you hit the "Submit" button, just scroll down a little further than the "Preview" and "Submit" buttons and you should see the Filename "Browse" and "Add the file" buttons - like this:
Then, you can upload a photo from your hard drive or whatever device you have it stored on. I hope that helps anyone who is unsure how the feature works. Thanks!