Hello,
I'm pretty new to Freelancer.com, so I don't have any reviews, but I would love to win this gig and show you what I've got. I'm a student, and I make YouTube gaming videos myself, so I know what viewers like to see in order to get the clicks and the views. A good thumbnail should be relatively high contrast with a large, clear image of a person of thing that can and will be seen in a large browser window.
If you would like me to provide a sample, I would be happy to check out your video and create the thumbnail up front. If you like it, you can pay me for the gig, and, if not, you can go with someone else. Please send me the link to the video and let me know if there is anything in specific that you were looking for. I use Photoshop and will provide a high resolution image.