I co-founded crowdSPRING. Illustration projects are perfectly appropriate crowdsourcing projects (we've done many for startups, big Brands and some of the world's best agencies). Let me offer a few suggestions that should help you get a good result regardless of where you post your project:
1. Be sensitive to your budget if the illustration is complex. Many people forget that illustrations can be rather complex - and you're talking about a large illustration (which might have many components and may need to interface with other on-page elements). You'll attract better illustrators if you can stretch your budget a bit (depending on complexity, of course).
2. Limit initial scope. If you ultimately need a full page illustration, you might initially ask for an illustration of a portion and then narrow your selections and ask a few people to show you more complete illustrations. Asking everyone to do full page illustrations - especially if your budget is modest - might limit the number of participants.
3. Identify styles you like. Even more important than sketching your idea is a reference for the types of illustrations you like. Many illustrators have a particular style and if you can be clear at the beginning, you'll help them focus on showing examples of work that you're more likely to like - rather than just take stabs in the dark submitting random illustration styles.
I hope these are helpful and wish you luck with the illustration (and your MVP!)
Ross