Bug1011460 - Fix wallpaper picker flickering

Fix involves separating thumbnail images shown in wallpaper list in a separate folder 'resources/thumbnails/' from the actual size wallpapers 'resources/' . As of now using high resolution images as thumbnails is memory intensive and causing wallpaper images to swap in and out. 

The PR is using 320x480 images in resources/thumbnails. If we go with this approach, we can request UX to provide images in smaller sizes for thumbnails.