Back again. I think that it is entirely possible to visualize and understand horror without images, given a quick mind and good imagination. I was quite able, as a young student, to grasp the inhumanity and carnage of the Nazi camps after reading about them, absent any photo images showing stacked bodies.
It is very hard to "unsee" an image, particularly for those with photographic memories. I would be very supportive of the teacher but agree with those who say this may not be fully thought out by same.