If you choose "thumbnail", you get a tiny image and have to click to see it. (Also, people will fuss at you.) If you choose "full size", the forum software will scale your photo to fit into the width of your comment. Click on the photo, and it'll expand to full size. If your photo is small enough to fit without scaling, then clicking won't do anything -- the forum software won't magnify your photo. Does this help? (Not 100% sure I'm answering the question you asked...)
I take the photo, crop it and then get the thumbnail. Before posting I click the full size and then post.