This seems...incredibly unfair.
Your 'contest' ran for six days. Well, not really RAN, but you had it open to submissions. You say you need 5 entries to even run the contest, but if you don't get five entries you won't run the it and no one gets 'paid'. So basically, you kept it 'open' to get free art, then closed before anyone had a chance to win anything from this.
This is incredibly unfair to anyone who entered your contest. You basically scammed these users out of their hard work and time; they never even made it to compete.
I feel sorry for these users and I really hope you consider paying them for their art.
Too bad; I'll have to auction off the piece I did for this.