Photos can be assigned in bulk either with face recognition techniques or importing multiple files at once (explained here) but in case you'd like to assign a specific photo to a single student you may do so from the seating plan or from the student's personal data screen.
From the gradebook, double-tap on a student
Now tap on the student's photo to change the photo, remove the current one or to take a new photo
Or from the seating plan view with a tap and hold on the selected student
Photo storage: When you assign a photo to a student, a low-resolution copy of the actual photo is used and stored. You can remove the original photos from your camera roll and the copies assigned to students will remain.
When you remove the assigned photo from a student, you actually remove the low-resolution copy and you'll still keep your original photo safe.
When you remove the assigned photo from a student, you actually remove the low-resolution copy and you'll still keep your original photo safe.