Alright, I figured it out. The problem isn't with the tool, it's the layout of your output folder. In your output folder you MUST have the folders 0bot, 1out, 2clo, 3fac, 4har, 5hat, 6tla, and 7tlb. If you remove those folders it will generate this error upon trying to save the packed image file. The layout of your input folder doesn't matter, you can use the original file structure included in Seliel's zip files, or throw just the .png files in there and it will work just fine. But in the output folder, you MUST have those folders present that I mentioned.