We found a very long PDF that goes way into depth on the topic of hiring illustrators to work on your projects. Knowing our readers are busy people, we've helpfully condensed the pamphlet into these highlights:
Plan your schedule, pad your budget and have clear expectations--ideally you’ll come to an illustrator with:
1. A studied view of their portfolio
2. A reasonable schedule
3. A respectful budget
4. Knowledge of the copyrights you need.
Illustrators are more than their portfolios suggest. Above all, look for skill. An artist who can draw well can probably draw well in more than one style. I’ve seen many artists go from one style to another and wow folks with their ability to transition. Illustrators may focus on one style for marketability, but that doesn’t mean we aren’t capable of other things.
When you commission artwork, you are paying an artist for two, maybe three things:
1. To spend time creating the art you require
2. For the rights to use the image in your product(s), and/or
3. Ownership of an original piece of physical artwork (a painting, for instance)
Some artists hand in extremely loose sketches. Others hand in very tight and detailed sketches. Perhaps this is something you should research on the artist’s website ahead of time. Seeing how an artist transitions from sketch to final will be helpful for you to be able to decipher what you’re given, and extrapolate to how it might relate to a final. Work with the illustrator to get the sketches right, but try not to micromanage your sketches. If none of the images suffice at all, ask for more. Depending on the work done to this point,
starting again may or may not require an additional fee. But don’t move forward until you’re happy.
The final art is a last chance to make some changes and, ideally, should require none. If you agreed to a sketch and the final is substantially similar, do not attempt to ask for a change to a part that could have been requested at the sketch stage.
While it IS long, it's a good read for those uninitiated in commissioning art. Take a look.