User posted a link to http://i.stack.imgur.com/g3aUt.png in this question. So I edited it to actually show the image. I didn't remember image syntax used here, so I used GUI button. that was much faster than reading syntax help and I expected it to gave me the same result as manual wrapping URL into image tags. I was surprised to see http://i.stack.imgur.com/CMZUc.png as created URL. I replaced new one with old URL and it worked good, too, so reupload wasn't really needed.
It seems each time someone will want to correct broken markup using GUI tools, new copy of image will be created. It's both a waste of time and HDD space. Could you add filter to GUI tools, so that if image is already on stack.imgur.com your tool would not try to upload it again, but would just use the copy available there?