@tsemmes – Have you looked at using a shortcode in a text widget to do this?
If you theme does not support shortcode structures in text widgets this article may be of assistance: http://wpfirstaid.com/2012/10/use-shortcodes-in-text-widgets/
~Cais.
Useful link: http://www.nextgen-gallery.com/nextgen-gallery-shortcodes/
Thanks. I wonder why the style edits I made don’t carry over to shortcode? I removed all the boxes around the thumbnails in the nggallery.css file but they are back again. You can see what I mean on the sidebar at http://www.tomsemmes.com/gallery/windows-and-doors/. I wanted just a simple thin rule around the thumbnail. And I had that on this page http://www.tomsemmes.com/gallery/
@tsemmes – You may need to be more specific with your custom CSS definitions for it to work with the sidebar instance.
– Cais.