Final Cut Pro Stylized Transition Question

Hi, I'm trying to use the built-in stylized transition.

It pans to the lower right, and uses images from a prior clip. Very cool. When I hover on the transition, it seems that there are drop zones; and each "postcard" is numbered. I just can't figure out how to replace the images chosen for me with ones I want. When I try dropping an image in, it instead adda the image to the timeline. And once perfected, can i save this custom transition? Please see illustration.