...what fred caro meant, zorin lite is supposedly a lightweight system, as in not using too many resources?
because KDE is the sheer opposite of that.
anyhow, using shotwell for this is not such a good solution.
chances are, there's already some other (more lightweight) program in zorin's repositories (ubuntu based iirc?) that does exactly what you want (desktop background changer/slideshow).
choosing the right program here is highly dependent on the desktop environment you use, and unfortunately i could not find any information on that on zorin's website (for the lite edu free version).
edit: it says gnome/lxde on distrowatch. supposing it uses lxde, here's a quick search result: http://ubuntuforums.org/showthread.php?t=2076417