Greetings,

I just installed ZP and started using it all of 10 minutes ago, and have a basic question about image navigation.

When I click on a thumbnail, a medium-size image is displayed, and when when I click on that, a full image is displayed. Is there a way to go back from the full image besides pressing the "back" button on my browser? If possible, I'd like to go back just by clicking again on the image.

No, unless you code your theme differently. An alternative would be to use the colorbox plugin. Depending on your theme use you might need to modify it as well.