Hi Fullernd and welcome to our community
One thing you may wish to try is to edit the image in an
image editor such as Microsoft Paint or Photoshop. Change the color
of the upper and lower left pixel to an odd value that doesn't
match the rest of the image. I'm not sure it will work, but these
pixels used to control transparent colors used in version 1. So I'm
guessing it might still have an influence in version 3.
Give that a go and let us know how you make out!
Cheers... Rick