When posting a reply, above the box into which you are typing you will see some action tiles, about in the middle there is a tile of a picture with a disk in front of it, hover over it and it will say "host an image", click on it and you will open a menu to browse for a file on your pc to upload. If your image is already hosted, or you have just grabed the url from your browser, then use the tile to the right, which just has a picture on it, and copy the url location into the poepped up menu box.
