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 grade book, 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 long tap on the selected student
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.