There are two approaches you can take. You can either take a snapshot with the client-side API or you can retrieve images on from the platform with our server-side API.
There is a small caveat with the snapshot technique: it creates an image from the current player’s viewport so if your user interacts with the model at all before taking a snapshot, that adjustment will be reflected in the snapshot. Be sure to either set a camera to position your item in the way you’d like or limit movement of the item in the platform.
I believe the server-side call may be your preference if you are trying to retrieve the fileId. You should not have to do any webgl renders since you already have images on the platform, this API call should get them to you.
Here are some posts that may help you, let me know if I can clarify anything: