I’m pretty sure that these are auto-generated if you write your code from a template based on the category you choose for your smart app to be listed as.
Is there a place where I can see all of the possible icon images (and http address links) so that I can choose the one that I like best?
Agreed. But don’t sweat. There are new ones coming right after they enable Bluetooth in the hub which is right behind the introduction of entry delay for SHM.
You can set your own image for a device by temporarily changing the device type to an arrival sensor which allows you to set your own image as the icon then you can set the device type back to what it was and I believe the image sticks. PIA but it works.