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Just refreshed the Server Fault main page and saw this in the right sidebar:
The image location emdedded into the page is http://i.imgur.com/LZEzv.jpg the alt text is Try OpenIndiana Today
Try OpenIndiana Today
Because imgur.com returns an http status code of 200 for that image, there's really no way for us to detect when an image is no longer available.
SvenW's action to edit the offending community ad was the proper one until its author can upload a new image.