Unfortunately, custom cover images for folders is not a feature that is currently available. I will submit a feature request, however, and if approved it may appear in a future release.
Edit: (attn: Caitlin Langhorne ) There is a partial solution for this. When editing a folder, there is a "Cover Image:" title near the bottom under the "Layout View" selection area. Beneath the Cover Image title, there is a selectable area. This is blank by default, but if selected will pop-up a menu of images located within that folder.
This will place that image in front of that folder when seen in the thumbnail view.
This feature is currently being worked on.