My understanding is that you have to first save the image to a program like Photobucket or Imgur. How to actually post it from there, I have no idea, sorry.
Posting Screenshots? There's a button (green) above my reply just about here that says [Insert/edit image]. Click the button to open a popup-window that says: Insert Image, Enter image URL:
Or you can copy and paste the BBC code from imgur. Thats what I do. Be careful to post the image BBC code, not a link to the page the image is on.