I have found that my attempts to attach JPG photos that have been modified using Photoshop result in this warning:
Your attachment has failed security checks and cannot be uploaded. Please consult the forum administrator
However, if I save the image as a GIF using the drop-down commands: File, Save For The Web ..., then everything uploads without problem. Its possible that if you use the Save For The Web feature and save as a jpg it will also work. Dunno. Never tried it. The GIF approach worked and I had a solution.
From reading the Simple Machine Forum for this SMF software, this has been a problem for several years. It can be fixed via changes to the background coding but that is often outside the comfort zone of many administrators.
Hope this helps someone.
