I do my thumbnails WHILE my video is uploading. My internet sucks, so I have plenty of time! lol I know for a fact that I need a 1280 x 720 frame. So I open photoshop. Make the frame, now it's time to put something in. Usually I'm doing a follow up to another video in the series. I probably have the logo for the series on the screen, I only need to change the background. So I'll go to my source footage and find a key moment which will look good in the frame and screen shot it. I'll make sure that it goes well with the title, and it is visually appealing. This is where saving your previously edited thumbnails comes in handy. You can do thumbnails fairly quickly if all you have to do is load the old one and replace the background image.
I have two 3 sets of established font styles, so I don't have to keep writing and resizing, it's already ready to go on the last thumbnail I saved in photoshop.