NO WAY
- just kidding - anyone should be eligible.
I think your biggest problem is how you keep competition going when someone guesses straight away. It would be better to reward the most accurate answer rather than just the first to name the city or the country. With the latter, you mostly reward the people who were online when you post the thread and could answer fastest (no offense to my good buddy Mouse and all those before me who robbed me of all the glory last time
). Also I think you need a system that keeps posted answers secret, so that interest is maintained for a couple of days. They could then be visibly added back in to the thread.
Hey, just my 2 cents
.