I bought my Kobo Aura in March and since then I carry it in a case. Nevertheless, a couple of weeks ago, the screen showed up much like as yours but only with horizontal lines at the bottom, just after I woke it from sleep. I don't know what caused it but I got a new one from warranty.
I got my cover from Gecko Covers at:
http://www.geckocovers.com/ereader-c...ra-6-inch.html