I can't tell you how many times I get asked about an author/illustrator team approaching a publisher with a joint project. It goes something like this. I've written a fantastic story, and my friend, neighbor, daughter, husband (take your pick) has done the cutest illustrations. Where should I send it? My answer is "NOWHERE!" That's just not the way it works in this business. Have you heard of art directors? No? Well, forget the story, and inform yourself about industry standards.
You might as well know that your chances of getting published with this approach have taken a serious nose dive. This sort of submission marks you as an amateur and gives publishers even more reasons to reject you. Remember, they have plenty of talented professionals to choose from. Besides, they may love the art but hate the writing. They may love the story but hate the illustrations. Or, they may like the story and even like the illustrations, but don't think they work well enough together. Or, they may have a totally different take on how a story should be packaged and it might just seem to messy to bother rejecting one half of the equation.
For a great take on this topic from an agent's perspective, check out Mary Cole's site.