Well, waste-not, want-not, as they say...
Also I read a (darkly) encouraging interview of Barry Blit, who is a frequent cover illustrator for the New Yorker, and he admitted to submitting around 10 rejected images for every one that was accepted. One year, he made it on 5 covers....and that same year, he estimated that he'd submitted 50 ideas. So he's a real genius and a stubborn one, I guess the two character traits go together.
What do you think?