Excellent! I've updated my distribution files with the fixed .png files so as others encounter the issue, a simple download/upload can fix the issue.
My main site is running PHP 8.1.4, but the GD library libpng is 1.5, so I'd not had the problem before. Lots of stackoverflow talk about the error with libpng 1.6+ versions. I had to install a portable version of ImageMagick and process the files.. glad it's no longer a problem. Thanks for bringing it to my attention, John.
BTW, the original images were produced by NOAA NWS graphic artists and captured in 2015 as they started changing to the new graphics icons.