My images start with thumbnails. There are a few exceptions here and there where I just wing it, but in the main I start with a thumbnail. I do pages and pages of thumbnails. Sometimes they're for a specific piece, sometimes they're just playing around. But I have an entire sketchbook devoted to tiny little thumbnails. In this case I did the thumbnail in pencil. I do most of them in pen, but there's a really comfortable feel that you can get from a pencil that I have a hard time finding in other mediums (Fig.01).
There are a few key things when you're working on thumbnails. Firstly keep it loose. A tight thumbnail somewhat removes the purpose of doing a thumbnail in my opinion. I believe thumbnails should be quick explorations of different directions you can take your composition. They are a way to play around with arrangement and orientation without a lot of time being invested. They're meant to be fun.
Another key is to do a lot of them, as many as you can force yourself to do (and then a few more). Very often the first sketch you do will be the one you pick, but just as often the twenty-second sketch will have something that none of the others did. Considering that each thumbnail might take you two or three minutes, it would only take you an hour to crank out a couple of dozen. Before you spend hours upon hours rendering a finished illustration, lay a good groundwork by finding a great composition first.
For this image I sketched out a quick drawing and scanned it in (Fig.02). I don't often work with lines because I have a more painterly approach to images. Some people do amazing work starting out with lines, I'd just personally rather dive in and paint some shapes. In this case I was looking to try something new and expand my horizons... and it worked pretty well! So let that be a lesson to all of us (especially me) - don't get too attached to your methods. Explore and try new things.
After I scanned in my line drawing I set it to Multiply on its own layer in Photoshop so I could paint underneath it. You'll note that my marks are broad and extremely general (Fig.03). I don't care in the slightest about details, texture, or rendering at this point. All I'm doing is considering the overall image. I even go so far as to force myself to not zoom in. It might not seem like much of a difference, but if you restrict yourself from zooming in you'll find that you simply can't pay attention to details. Give it a shot, you might find you like it.
This stage is about 30 minutes into the piece. Already I've established the general values, colors and lighting. This is the part of the painting where I start to feel a bit more confident and if I squint really hard I can sort of imagine what the end result will look like. That's my goal for every piece I paint. In under an hour I like to have a really good idea of what this image will turn out like. If I can't then I'm probably not focusing enough on the big picture.
This is the first pass of painting on top of the line work (Fig.04). You can still see some lines poking through, but now I'm starting to really paint this thing. Where before I was treating things very abstractly, here I begin to define the structure of what's going on. Most of this piece is rocks and snow, so I can be pretty free with the size and shape of things, but I still need to keep in mind how light interacts with all of the objects. Which brings me to a good point: lighting. How I light pieces is one of the most enjoyable aspects of paintings for me. There are so many varieties in lighting situations that can be used on any given piece and all of them will give off different moods. Selecting the one that gives you an interesting arrangement of values as well as conveying the correct mood is a challenge in and of itself. I see so many artists who seem content with standard overhead lighting on every single thing they paint. I personally find this incredibly dull. My best advice on how to get better at lighting would be to study master paintings and movies. Check out the work of Albert Bierstadt, Thomas Moran, and George Inness and you'll begin to see how to make a painting drip with light.