How to create TOC style icon files to use with Option startup screen
This is a project I looked at some time back.
I’m pretty sure you need to start with the 1024x1024 size, and the system will scale it as needed.
There are programs (apps) out there like IconEdit etc, although that one may be a bit old now, that creates the proper file type.
Edit: I’m pretty sure the images or at least their names are stored in a control file that is read to find the appropriate image.