If you want to insert into a post......then ...from ANOTHER SOURCE, copy the url, then click on the tree icon in the line above the post, insert the url, and then click on the "insert".
If you want to insert into a post from YOUR OWN COMPUTER, then click on the "upload a file" button . And by the way, after that appears, you either have it as a tiny pic, or "full image", which you click on afterwards, and before you post your reply.