Hi, I was able to find what was causing the issue. I guess there is a minor bug. 🙂
In the main settings I had set the “<b>Featured Image Resize Quality:</b>” setting to 75.
For some reason, the imagepng function at line 336 of /res/ImageResize/ImageResize.php is using the value of 75, which is invalid in this case.
Hope that helps!
Stefano