In your UserCP, on the left is a pictures and albums pickoff. Select it, then add an album. Click on the upload picture button and load your photo.
Once it is on this site, open the picture, right click on it and copy the photo URL. Go to the thread you want to add the photo to, and click on the button above that is a square box with a mountain and looks like a yellow sky (image button). Right click in the box that opens and past the URL. Click ok and your photo should show in your post.