Kirby....
This has been going on for a while. See the discussion here:
http://forums.prosoundweb.com/index.php/topic,134741.20.html
A little more info: A trigger for this appears to be when people try to attach Photoshopped images which have xml with links to other websites in the metadata. But then we can't expect people to edit their xml data.
Or can we? If you use Save for Web & Devices, the xml data is stripped down to just a few lines, which may well explain why in the above referenced thread I was able to post a jpg after saving for web & devices that I could not post before. Something in the xml data is triggering the security check fail. So Kirby, try Saving for Web and reposting.
There is a way to turn off image checking security altogether, but doing so could expose the site and server to possible exploitation, so I'm reluctant to go that route unless necessary (and I have not tested it).
