After you paste the picture in Paint, you need to save it, such as to your desktop.
When you write your reply on the forum, click the paper clip icon (see below), then click the first 'Browse' button and double-click that file you saved.
Once uploaded, close that attachment window. Place your cursor where you would like to insert it, click the small down arrow to the right of the paper clip icon and click the file you attached. This thread goes into more detail if you've never attached a file before.
Once you submit your reply, your screenshot should hopefully appear in the post.