I created this image as a solution to an assignment I gave my UVU illustration class a few weeks ago but that's another story.
It's important to remember that illustration isn't photography. We don't have to play by all the rules of the natural world. We are creating an illusion. Our job is to communicate an idea -so bending and/or breaking rules is ok as long as it works and as long as we have a good reason. We can add atmosphere and lighting where we need it to set up a shot - similar to how it would be done in film. Have you ever noticed that night scenes in motion pictures often seem much lighter than they normally would? That's because it would be really boring to only hear sound while watching a dark, almost black night scene. Directors bend the rule of darkness by adding light from places where often there wouldn't be any light. Cheating? Of course!
I decided to use the 3 types of light sources you can have in an image. 1) On camera 2) On camera hidden 3) Off camera.
You can see below that using the candle alone creates a very dark mood. I wanted this illustration to be playful so one light source wasn't going to be enough. I wanted my viewers to see lots of items in the cave.
Adding a second light source behind the pirate helps by illuminating the right side of the cave. I envisioned this as perhaps a lantern that is below the pirate and objects in the foreground (on camera hidden.) These two light sources just aren't enough however to define the cave opening.
I had to add a third light source (the moon - off camera) that would shine down and reveal the cave entrance and the pirate ship on the horizon.
And this is what it would look like with only the two lesser light sources - moon and lantern. You have to have a primary light that helps create your focal point(s).
It's much easier to use one light source in your illustrations but in certain situations you can achieve the results you're looking for by adding multiple light sources as long as you make one your primary and the others secondary.
If you're looking for more instruction on Painting Color and Light we have a video course at www.svslearn.com that might be what you're looking for.