Well played,
@mashers. Checking the log.txt, I see the following error:
[process_file] color_type of input file must be PNG_COLOR_TYPE_RGB
However, when opening the images up in GIMP, it does report that they're RGB.
I was able to get it working by copying the images out of GIMP and pasting them into MS Paint and saving them there. Opening the images in Paint and re-saving them that way
didn't solve the issue, it was indeed necessary to copy the images to the clipboard and paste them into Paint. Regardless, I assume this is some kind of fault in libpng rather than anything on your end. Thanks for the help!