Is it bad to use an old picture of yourself for your profile? I’ve gained some weight recently.

That seems a little wrong. If you are not expecting to meet anyone in person it might be ok. But this is bait and switch.

You've got to be honest if you expect other people to be honest w you. Post a current pic and represent yourself.

be yourself!!!

I don't recommend to use an old picture, because you may have changed since the photo was taken .. if you don't know what's your best photo to put in your profile, get a friend or family member to help you out!