There are some improvements in the works for the themes screens in 3.4 (see #19815, #19816, and #19853). #19816 needs some help from the UI group.
Ideas proposed so far include having a slider inline with the current placement of the screenshot (not great, linear nav only can be frustrating), a photo stack that displays in a light box when clicked, or a div/layer that slides open to show the screenshots. If screenshots can be included in larger sizes as opposed to the current 300×225 size, which is actually squeezed down to 240×180 by CSS in the admin, a lightbox of some sort would be nice.
If anybody has some ideas, please post them and/or comps in the comments. Note that the themes screens will likely look more similar to those on WordPress.com, with most details hidden and a real focus on the screenshots, title, and author – working patch on #19853 if you’re so inclined.