I used the Add Featured Image link to add a bunch of images to my most recent posts so now the category pages have a thumbnail for the first few pages. Thankfully I had kept all my thumbnails organized by the month I created the post so really wasn't that hard to add them in.
I had used Thumbnails for Excerpts plugin for awhile but to get it to work I had to include an image in the post which is what I was trying to avoid. It's a good plugin but unfortunately it hasn't been updated in a long time and that always makes me a little leery to use it.
This will be easy to do going forward so thanks again for your help in pointing me towards the featured image function.