Most sites will have some good choices of illustrations. It doesn't have to be anything special. I really think you could get away with glorified stick figures and still make a great video.
The script and storyline is important. The big takeaway that I've gotten so far has been to make it "story-based". So instead of just a screencast saying "Crowd2 helps you create a marketing campaign to advertise your products", try, "Kevin is looking for ways to tell people about his new CD release..."
