[Andrenid]

I think you should be able to pay as much as you want. Each dollar you pay* = 1 minute-credit. So say you pay $50, you get 50 minutes. Someone then needs to outbid your remaining minute-credits to replace you.

Eg if you paid for 30 minutes ($30) but it has been up for 15 minutes, you have 15 minutes remaining, so someone has to bid $15 or more to replace you.

Once you get to 0 minutes left, your picture stays up until someone pays $1 or more to add their own minute-credits and replace you.

This 'decay' means that you'll always have people trying to snipe as soon as it's affordable, to get their pic up.

Have a timer underneath saying "5 minutes remaining..." to give people an idea of what they need to bid over to get their pic up.

(*The actual price suggested is just random to make it easy to explain. $1/minute is probably too much in reality)