I've been able to connect to the Nest API and correctly get information about my Camera, however, there seems to be no way to get a static image from the camera. My user account does not have a subscription. I'm not looking for past data as I know that is not available for a user with no subscription, but as the camera is live online, I was surprised I couldn't see anything in the API for getting the current image.
Currently there is no way to capture a live image on demand. Images and animated gifs are available when events are triggered, but as you noticed are only available to cameras with Nest Aware subscription.