Although it's not free, it is low-priced as is definitely worth the money. I really enjoy and recommend Affinity Designer. I used to use Illustrator, and although I still have it, I actually prefer to use the cheaper Affinity Designer.
For those of you who are more retro gamers, piskelapp.com is a good pixel-art design software that's free. I've used it in the past and it works like a charm! It would work for thumbnails, but it would mostly be for animated graphics, since it's so easy to animate with it. It's so easy that even someone with no experience like me can use it to make animations such as...
A thumbnail will take a few moments and an animation such as this will take only a few minutes for an experienced designer. It took me about an hour to make this one though, due to my inexperience, but it was not an unenjoyable experience.
I make my art on Paint.Net it's a free service that can be very useful. I suggest getting the plugin pack aswell because it comes with many awesome features!
I really do like making art and it is awesome that i can do it with a great service! CHECK THEM OUT!!!