What is the difference between image and image rectangle?

The Image object is for static images that you don't need to manipulate later in the API. This is useful for logos or branding that you want to keep the same in all of the generated images.

Image Rectangle and Image Circle are dynamic objects for creating image containers that you can change later via the API.

Static Objects have limited modification while Dynamic Objects have a multitude of modifications and enhancements that you can apply (e.g. Smart Crop, Face Detection).