This is really confusing and fact they don’t let you name the camera on creation. One of the things I missed in the updated readme which I will add now. Added!
In the Generic Camera → Still image URL box put:
http://<moonraker-ip-address>:7125/server/files/gcodes/{{ states("sensor.3d_printer_object_thumbnails") }}
Make sure to read the Note at the end of that section there as well about thumbnail quantity.
Let me know if that works.