I was wondering if there was a way to insert a picture of the add on in the place of the generic unknown thumbnail that appears with some addons.
Thanks
Sure is. Generic unknown generally means a thumbnail wasn't included. At least that's my understanding of your question. You can make your own thumbnail with a picture viewer like IrFanview. Look at some of the thumbnails listed under texture of another AC to see the width and depth of it (ex. 256 X 190).
Take a screenshot that you want to use and resize to about the same size as what you may see with another thumbnail. Save it as a bitmap to the texture folder of the aircraft that you want. Some AC/Helicopters have more than 1 texture so be careful where you put it.
You now have a nice thumbnail you can use to identify the AC you want to fly.
Hope that helps.
Edit: Hells bells, Brett, you beat me to the punch

And yes, you are right - save it as thumbnail.bmp.