Joshua, only you as the GM can see the page toolbar, including those preview images. Is this a problem because you and your group are sharing the same computer? Is it possible to put them on a 2nd computer, so that you're using yours as a GM screen? If not, the page preview image is the largest art asset on the page. If you really needed to obfuscate that you could upload a transparent image and re-size it to be the largest image on the page and that would change the page preview to be nothing.