Honestly, you'll just have to fiddle with it in every way possible to get it to work. GIMP works best for this. Here are a few steps you can try.
Open it, save it as .png, then close and reopen and save as .jpg (or the other way around)
Change the Image mode from Greyscale to RGB, save, reopen, go back to Greyscale
Go from 513x513 to 512x512, then back to 513x513
Copy the image to the clipboard, paste into empty GIMP window, flatten and save
I know you've tried some of these, but sometimes you can get it to work after a few combinations of this.
You are still welcome at any time to send it to me and I can try some other tricks as well.
