i think 0.05
BTC is a good prize for an avatar it is nearly $22 at the current rate.
and also for your contest you should specify what kind of avatar you want, it can be a drawn picture, or sometimes a simple picture of a character can become an awesome avatar with some small photoshop work
• Any member above Jr Member rank can participate (losers make multiple entries with newbie accounts,maybe you can only allow Member+)
•Allow only 2-3 maximum entries by each member so they can bring the best out in less time.
why does these two matter?
i think it is not important at all who enters or how many times they enter, it is not a giveaway or lottery game. if the design is good they can submit 1000 avatars as long as OP is not picking by chance but picking by quality.