I have similar issues, uploading jpg images give errors that the file is in the wrong format and when I export the same image to .gif then upload it, I see no addition, just the pic.
Could be the size exceeding some limits? I am testing at
this site
Would it be possible to have an automatic resize to a maximum size (horizontal or vertical) when adding new adoptable?
Also, I assume that script does not work with animated gifs or it will stop animation, am I right?