These stories are always so depressing.
On the one hand, you would definitely want the teacher to have addressed whatever topic they were wanting to more sensitively/tactfully. If they were going to show the image, they ought to have brought up the potential for this with their class first; the inevitable objections could have lead to the sort of interesting debate on the subject in a more healthier manner than any that would have come from showing the image at all. The politicised fallout on both sides just adds to the circus, and takes away from any potential productive discourse/resolution.
At the same time though, ultimately, there is something abhorrent in the idea of any religion dictating their views so vehemently in a school/public/secular setting. If you are in support of a diverse, open-minded and accepting society, both historically and to this day, fundamentalist religions have always been, and remain, the greatest threat facing this.
My partner is a teacher in a very diverse part of London and has had issues contending with certain issues against the backdrop of fundamentalist religious communities, particularly around sexuality. She would never do anything as inflammatory as what this teacher has done, but she's found herself in a very uncomfortable scenarios where large portions of her class have openly rejected and criticised literary portrayals of certain relationships/individuals on religious grounds. Which is absolutely terrifying to anyone who happens to be a member of these communities that have always been the most marginalised/attacked, let alone for the students who are having to contend with identity issues of their own whilst in these environments.
Intolerance in all its manifestations is the greatest underlying threat to education, which demands an open-mind to people/communities/ideas in order to flourish. From whatever side, it ought to be confronted through open-discourse.
In this case, it's hard to see how anyone/group is in the right. Both will demonise the other leading to further distrust and disharmony in the communities impacted.