How To Automatically Assign Images

I have moved one of my websites, WellBuzz.com, to a new theme and it seems like the featured images are no longer appearing for some posts. For example, if you go here: http://www.wellbuzz.com/page/2/ you will see there are images missing for some posts. We have thousands of posts, so manually going through and setting the first image in the post to be the "featured image" is not a feasible solution. So how can we automatically have the first image in a post be the featured image for all of our past posts that do not already have a featured image assigned to them?

I have granted wpmu support access via the wpmu plugin for wordpress, in case that helps.

Thanks for your help!