The incentive to deposit money is being able to actually guess the picture and claim the prize.
Naw, depositing money gives everyone an equal chance to guess it. If you want people to deposit money, it should be a strong personal incentive. Like in bitcoinduit, depositing ฿0.1 for a chance at ฿5.
Consider changing to this setup:
Start out with a picture with ~฿0.20 already deposited
Accept guesses only for a donation of ฿0.05
This way, every guess makes the picture easier, but it is the only way to guess.
Is it a feather boa?