I checked latest beta and this problem still appears, but now after some test I'm not sure if it's bit depth dependent or it's caused by something else. Simplest way it would be to test ShoeBox packer on attached files:
- base file is bitmap font (24bit PNG) from DingsFont tool, it will cause error,
- black image sized same as above file, saved/exported from Gimp 24bit PNG, will cause error,
- same black image but saved from Paint.Net, wont raise the error,
I hope that this will help somewhat to find the issue, if You prefer You can send me only 'ShoeBox.exe', as there is no need to do all those Steam app update stuff before it's fixed.