Well, we could do that, and it wouldn't even be that hard. But the toolbar has the useful slide show start/stop button, permalink, and show image metadata buttons. If anything, I could add a config setting that hides the download button for anonymous users. But all you have to do is right-click the image and choose Save As to defeat it, so it hardly seems worth the effort.
Remember that one option is to apply watermarks to all images unless you are logged on and a member of a role that has the "do not show watermarks" permission.
And there is also the config setting enableMediaObjectDownload that, when set to false, hides the download button as you request (but it does this for everyone).
Roger Martin
Creator and Lead Developer of Gallery Server Pro