"Your attachment has failed security checks and cannot be uploaded.
Please consult the forum administrator."
This is a known bug in the forum script.
From what I hear, it`s the way certain programs like older versions of Photoshop places non standard data into images.
It seems to happen with mostly older images and some animated gifs.
Although I have run into this while testing, I don`t run into it much. I use Photoshop cs6.
After messing around, I found that a workaround is to upload the image which did not pass the security check to your gallery.
Then paste the BB code into your post.
Your image should show up.
Sorry for the inconvenience