We've started caching forum images for various performance and security reasons (yeah I know it was buggy for a couple days). We download the image once to the cache, meaning if you need to do an update to the image, you'll need to give it a different name/url.
That is, if you make post with the image http://suck.it/my-crappy-design.jpg and do some changes to the design to make it less crappy, you'll need to edit the post and change the image url to something like http://suck.it/my-crappy-design-2-the-new-batch.jpg for the cache to download the new version.
"O-M-G, this is so annoying" you say. I know, but we live in world where people do bad things if we let them host images off of creepy Bulgarian web servers.