Two options, both available to all subscribed members:
Public model image library: My Products → Actions → Edit listing → Additional product images. Select images, add them to "My product images," then Sync to store. New model images for a product are flagged with a small red dot on the My Products page.
Custom AI model generation: My Products → Edit listing → the "+" button at the bottom of the image list. Generation takes a few minutes (you can navigate away); once satisfied, add the image to "My product images." Subject to your monthly AI credit quota (see What are AI credits and how do I get more?).
Models in both the library and custom generations are AI-generated.