Thumbnails are great for all types of art and are used by every artist. But what exactly are thumbnails, what are they good for and why does everybody use them?
As you can tell by the name, thumbnails are small sketches done quickly and loosely to visualize any idea in a short amount of time.
In illustration and therefore concept art, it is a way to nail down ideas without having to worry about stuff that is unimportant in the early stages of design, such as details.
You get a better overview of the whole image when doing thumbnails and therefore you are more aware of composition, focal points and where the eye is lead. Furthermore, you can bring much more ideas to the canvas than you would doing it in full size.
Whenever I make illustrations for clients, before sketching or painting anything, I quickly do one thumbnail after another. Instead of trying to be perfect in the thumbnails already (artists tend to try to always perform well…), I really just sketch what would be my first try, without thinking about any fundamental stuff. But since that sketch is so small and done in a few seconds, I can then put in more and more thought into the process. That way, I get from a thumbnail that lacks fundamental illustration aspects to a group of thumbnails that are getting the job done, and that in a relatively short time but with huge effect on what the finished work will look like. Then the rest is so much easier, you clean it up, you paint and boom.
Always be open to new ideas coming up during the process. You will benefit a lot from getting into the habit of thumbnailing EVERY idea you have. Your thumbnails will get better as you keep improving as an artist, which in turn will make your designs better.
Here are some examples. Thumbnails can be helpful for: