I saw the following edit suggestion today:
The original poster included an image with URL
http://i.stack.imgur.com/SeZRb.jpg but didn't use the proper formatting to inline the image because his reputation is too low.
A helpful editor hit the "image" button in the editor and pasted in the URL, and probably the imgur-uploader tool dutifully downloaded and then re-uploaded the image to imgur.
I've seen this behavior when editing posts myself but just worked around it by typing the Markdown manually.
I'd like to suggest that the "image upload" button on the toolbar simply add the formatting around the input URL if the input URL is already on
i.stack.imgur.com. This change would make reviewing suggested edits easier.