Look at the top of the forum page, and there is a Upload link. Also every time you respond to a message, look above the post (before you post) and you will see an Upload Your Image link. That is where you do it. And you can put it in a number of folders.
Just make sure, before you post your image, edit the description and title. I keep forgetting to do that, and I don't think you can go back and change it after...