The problem I see (other than the fact that people are not too likely to take this offer) would be the question of: how can we know that the weight loss is real.

You could have hypothetically taken a picture 6 months ago and have lost 75 pounds until now. You could then just show the pictures and boom, win.

Secondly, super long term bets of like a year are again, unlikely to be taken. One party is likely to leave before the time span.

If you really want to incentivise yourself to lose weight, try stickk.com. If you don't lose the weight within the timespan, your money will automatically be donated to charity. You'd be betting against yourself, and still get the motivational benefits of having your money on the line.