Dietbet?
I've participated before. Given how you have to take the pics, it would be fairly hard to cheat. You can't lose TOO much, or they'll disqualify you for not doing it a healthy way. At the end, they send you a code word that you have to show in your pics (one of you on the scale and one of the number on the scale). I've really enjoyed it when I've done it in the past.