Yes, you can select art for the poster and for the artwork that would appear behind the poster when a poster is highlighted.And it can be done for Poster and Fanart seperately ?
See the images I posted. Both the poster and backdrop have the option to add your own artwork.
Note that when selecting the artwork after navigating to it that you'll not be able to preview it until after you've selected and added it. Don't expect to be able to browse the arwork as stored on drives using the poster selection option.