This plugin doesn’t appear to have its own stylesheet, but using Firebug I found a class of .ngg-sidebar-widget and was able to add a border to my image. I’m using a child theme based on twenty-eleven, and this is the code that worked for me, just added it to my child’s stylesheet:
Thank you. I was able to solve the issue. My theme has a nice simple form for adding additional CSS tags. I’m not sure where it’s putting them. However, after a few tries to be sure I named the proper class, I got it.