You are correct that you can't get more than one image.
Unfortunately as it's currently in the API as an object rather than an array, it would be a breaking change (for anyone currently using the API) and so it's not likely that this will be added to the current endpoints.
If it did get added, we'd have to have it as a separate endpoint to avoid this.
Adding this (as a separate endpoint) is definitely intended for the future. The reason it's not there yet is because I don't know if the current image specifications (e.g. the banner/poster format) are stable enough to release in the API.
The more interest in it, the quicker it would be added. :)