Definition: "Thumbnails are reduced-size versions of pictures, used to help in recognizing and organizing them, serving the same role for images as a normal text index for words." (Wikipdia)
Now, that doesn't sound like it has something to do with art, does it? Well, let's use them anyway! 
What? Plan a picture before you put too much work in and start going into the details.
Why? Because everyone of us has been at that point of "YEAH, I have a BRILLIANT idea" and jumped into work, only to realize with sudden frustration that they just spend several hours on something that looked like ... well. Nothing you'd publish.
Going back and changing half of your composition sucks, especially after you already worked on it for 10h. Planning everything before you start the work helps you to figure out if things will actually work out the way you imagined. Things never turn out 100% how you had them in mind, so putting them on paper gives you a clear goal to work towards and the reassurance that there is a way to display everything and create the emotion/effect/... that you're aiming for.
Also, picking stock is a lot easier when you know exactly what you're looking for.
When? You can use this technique whenever an idea comes to your mind or right before starting a manip. I do it, whenever I'm about to start a photomanipulation, in order to visualize what I'm about to create.
Doing the planning afterwards doesn't make sense, obviously.
Who? Everybody who wants to convey great ideas and is willing to put 10-30min into planning to save time and frustration later.
It's also great for collaborations, because you can talk about stuff and get the same idea of what the heck you're about to start.
How? Planning is what you do, thumbnailing is how you do it. Think of it as a better way of sketching. Your goal is to produce multiple, different set-ups for a specific idea at a smaller scale.
1. Sketch the scene very roughly.
2. Shade it. Very roughly.
3. Put colors in. (Do I need to say it? Again, very roughly.)
> Don't zoom in, you want to keep an eye on the entire composition.
> Details are not important. NOT. IMPORTANT. Same goes for anatomical accuracy for example. It doesn't matter right now.
> Don't spend too long on one thumbnail. A couple of minutes are enough.
> Make more than one thumbnail, just to be sure you've thought about other (and maybe better?) ways to depict it.
> This is also the point, where you can think about composition and lines. There are a lot tutorials on dA, go check them out if you're insecure about that topic.
Finally The one thing people keep telling me whenever I bring up this topic:
"But I can't draw!" Doesn't matter. Seriously. Nobody is ever going to see one of your thumbnails (except if you show them), and a face doesn't need to look 100% like a face. It's important that you know what it's supposed to be. Draw an egg instead of a head and shade it, more is not needed. Trust me.
If you want to challenge yourself to get a little practice in, you can check this out: ArtCompositionChallenge|TASK 1-5|NEW: 4 + 5
