Im sorry for the inconvinience CuriousCD.
From what I researched, it`s been a bug for years, hopefully they will fix and update the script.
The solution may be another inconvinience too

The fix is the same solution as I posted for uploading animated gifs. I remember getting that error, when I tried uploading certain animated gifs and PNG files which had different coding than normal.
I actually could not find a fix online, but I did tinker with it and figured out a work around.
The gallery uploader is a not part of the forum script, so it has it`s own security rules that do not have to abide with the forum`s security.
Upload the file to your gallery. (Every member has their own personal gallery here)
As soon as the upload completes, there will be a BB code under your image.
Just paste that code into your post and your images should appear.
Please give it a try, and let me know if it works.
I apologize for the inconvenience.
Thank you again for sharing your wonderful captions!