Usage question

[[J4F] Sentinel]

Greetings,
I've bought and tried card engine, but I can't seem to find a way to do one thing I would have expected to be there in the template editing: masking layers, or, at least, limiting an image within a shape.
For instance, let's keep it simple, and suppose I have an artwork that I would like to crop/resize within a circle shape.
If said artwork and shapes are added to the template, i could not find any option to do so. Neither if they are in different layers, nor if they are in the same.

What is the correct way to proceed?