Just tried something else. Instead of just picking Hercules from the list, setting it up with the new png files, and trying to save it using save image I clicked the Add button first and created a unit I called "whatever". When I went in and set the image to be from the Hercules folder it let me save it successfully.
The problem may have been I copied everything over to the Hercules folder including the _ImageDesc.unit file. Maybe this made the editor think it was an already existing unit but that there were conflicts between the data in the .unit file and the files in the folder and this was causing the error when I tried to save.