Basically you have to take a picture of your avatar smiling like they are posing for a yearbook picture. Yearbooks aren't a thing in the UK but we do have photo day where your parents get to buy a bad, overpriced picture of you against a background of fake books!
I had fun taking this pic. I used the AnyPose Expression Hud to make my avatar smile and then photoshopped a few images together so my eyes were less closed. I think it's best to just take a few pics and fiddle around until you have a picture you like the look of!