[racket] Doing collision detection in universe.ss

I've been weaning my son off of Scratch in favor of Racket, and trying
to get him to write interactive games using universe.ss and image.ss.
I'm wondering if anyone has suggestions for how to do things like
collision detection.  image.ss has these nice first-class images, but
I don't see a good way of querying two images to see if they overlap.

Has anyone else had luck in doing this?  universe has a nice
programming model, but I've found it challenging to find simple ways
of doing the kinds of things that Scratch makes easy.

y