Go to the Microsoft symbol in the bottom left-hand corner and click that. Enter %appdata% into the search bar. Then, find the file entitled, "Minecraft". Open that folder up, and you will find a folder that says, "Screenshots". All your screenshots will be in that file. Drag them out of that file and into a new folder. From there, you should be able to insert pictures onto the forums via the "image" button in the text box.