That's the point of this set! It's a sketch edition, letting you and your friends draw your own Munchkin set.
Any time someone goes up a level, they can illustrate and sign a card.
Go to a convention and ask every artist you meet to draw one card. Date the pictures! (Please don't ask an artist to illustrate the whole set. Unless you and the artist both think that's funny.)
Hand it to your kid. Maybe you're raising the next Rembrandt and every card will be worth thousands of bucks!
Pick a master of modern art and make him weep in his grave!