To modify the icon of a device via the API of PRTG, you can use the following API call:
Please, change the "variables" in the request according to your configuration.
You can find the name of the pictures available by default in PRTG under C:\Program Files (x86)\PRTG Network Monitor\webroot\icons\devices.
You can also find the name by using the Inspect menu (F12) of your browser under the "Settings" tab of the device:
If you want to add more icons, please use the same folder.