I would take a look at
Youtube Thumbnail Art & Photography because there are examples you can look at there, and you could also get opinions on which thumbnail to use out of the thumbnails you have created. Here are some general tips though:
1. Try to stick to a theme with the thumbnails on your channel
2. Make sure all text used on the thumbnail is readable
3. Choose colors that go together and stand out
4. You can use websites such as
http://pixlr.com/ to make free thumbnails, but they also have a good amount of tools that can make thumbnails look nice.
Here is a thread you can check out that will have some other tips too:
https://community.tubebuddy.com/index.php?threads/39174/