well avatar bets are not set in stone. you are pretty much taking the other person's word that if they lose they will man up. in an avatar bet, the winner gets to pick the loser's avatar and the loser has to keep said avatar for a certain amount of time that would be specified in the bet. if they don't go through with it, they will be ridiculed by every member on here so pretty much everyone goes through with it.
and to get a gif as your avatar, you have to make sure that it is smaller than size limit. for premium members, the limit is 240kb, but for regular members it is 40kb if i remember correctly. if you want a gif as your avatar, go to the MMA Avatars
thread and request one and one of the nice members there will make sure it is within the size limit for you. then you just save it, and then select it as your avatar.
hope i helped