People generally want to look attractive in photos, and a face that looks slim helps to achieve this goal. To create photos where your face looks as slim as you desire, there are several positioning and styling tricks that you can use.
Format the lighting so that it highlights only your face, like a spotlight, and the rest of the picture is in shadow.
Turn your face away from the camera so you are not looking at it directly. If the camera is angled so you are facing slightly off to the side of it, it will help your face look thinner.
Look up to the camera instead of down to it or directly at it. This will mean either lowering yourself below the camera so you are looking up at it, or positioning the camera higher than your head.
Extend your chin and jaw out instead of tucking it in towards your neck.
Add volume and highlights and low-lights to your hair to keep it from hanging flat around your face. Monochrome hair that hangs limply around your face will make it look wider. Adding touches of bright color and styling your hair with gel will allow you to craft it so it curves in and out at all the right places.
Practice your face and head positions and angles in a mirror prior to taking any photos, so that you become aware of the most flattering poses.