Easiest is to either right-click on image and select "copy image URL" or "open image in new tab" and copy the URL from the address bar there. Then, choose the little "sun and mountain" icon above the response window and paste the URL in the URL box.
If the image is on your computer, you can simply drag'n'drop the image into the response window, and then select thumbnail or full-size. If you have several images to post this way, drop them all first, then select thumbnail or full-size for all of them at once..