Start by learning how to use Photoshop CS6, this program is extremely good for things like thumbnails, profile art, etc. If you can't afford it, there are other "ways" you could obtain it, but the safest way is to just normally purchase it. Once you're familiar with all of it's tools and how to use it ( might take a long time, i've been using it for around 3 years and now i fully know my way around it ), just keep making thumbnails and they'll get better with time. If you look at the thumbnail for my first video then compare it through all of them to my current one, you'll see how much they improved in just a 12-ish video span. This isn't because I looked up tutorials or anything, but each time I made a thumbnail I could just think of ways to make it better than the last. Keep at it and soon the'll be looking great.