Board Thread:General Mod Discussion/@comment-26168781-20150410150940/@comment-25869554-20150410151431

go to the search bar at the 'start' menu (sorry if you have a Mac :) and search %appdata%. Open the folder, then open roaming then open .minecraft. Go to screenshots and find the picture you want to add. Copy this picture to an easily accessible place such as the My pictures folder. Then, when you upload a picture, it will be there.