I just use the good old paint program that comes with every windows, but u can update it to the newer version paint.net it works better
then just find a good screen from your video and make it interesting.
I am still trying new things, but I use an app called canva. It's is ok if you just want to place some text in front of a screen shot of your videos. It also has some other options like shapes and things to help get your thumbnail to pop.