Go to resource packs. Then click "Open resource folder". Exit full screen (drag minecraft to the side) and there will be something that has a list of minecraft related items. Find resource packs and click on it. It will show you all of your screen shots. Then choose one and drag it into the area on the forums where you are going to post and it will upload. Then it will let you choose if you want it to be shown as a thumbnail or full image. That's how you upload a photo.
Okay I got it, but it says that the folder is empty when I know I took a screenshot so this is even more confusing.:baw:
www.imgur.com is a good picture uploading site. I'll close this, seeing that the question has been answered.