Within the Student Profile, a character's portrait is displayed. They are .PNG images drawn from the Portraits folder in StreamingAssets. These can be edited with any image-editing software. The name of the file corresponds to the student's ID number, for example 'Student_1.png' is the portrait of the student with an ID of 1.
For best results making a custom portrait, pose a customised character in the Photography Club room in front of the pink gradient backdrop.
The custom portraits appear to be taken from the Photography Club room, in which there is a pink-gradient backdrop matching that in the default portraits. Sadly, the collision data in-game prevents custom portraits being taken.