As far as I know there is no way to add your own thumbnail. I also have to name them carefully to know what they are.
The one's I'm not currently using often like hoilday ones, I'll add a Z to the title that way they are at the end of the list. When the holidays come around and I need those I either just remove the Z or add 1 to the title to bring it to the top.
Not a perfect system but it works for me