Ok, this is how to do it.
Go to the thread you want to make, where it says upload image/thumbanil, click it and the box should appear. Go in the top search bar (The long one) and search %appdata%. Alot of files come up and you want to click onto minecraft and then in that file, click on the screenshots. Then all you have to do is double click it then the open option should appear on the bottom right corner. You have successfully uploaded it :D