[Plugin: NextGEN Gallery] Putting background image behind widget thumbnails

I’d really like to have random images in my sidebar and I thought it would be better to do it through a gallery plugin than just an image rotator plugin.

I made a background to the right size for my sidebar, and made sure the thumbnail was small enough to show the background image, but it seems to not appear.

The background image is here:
http://squirrelyknits.com/wp-content/plugins/nextgen-gallery/photobg.png

You can see the plugin at the bottom of the sidebar on my homepage:

http://squirrelyknits.com

Any help would be appreciated!

Hazy