You can use Adobe Illustrator if you can afford or you can use Inkscape which what I used when I first started and honestly its more than enough.and its FREE!
Make sure your thumbnails are eye catching and related to your video. I usually use an interesting scene from the video its made for and I add my channel logo to a corner that is not the bottom right corner because usually its covered by the video time length.
I would take advantage of the masking and clipping to create a unique thumbnail template for each playlist you create on youtube.
Take a look at my YouTube to get an idea.
Good luck!