Heartrending means so sad that it hurts to watch. The photograph you have to look away from.
The image inside the word is violent — to rend is to tear, so a heartrending story tears the heart. That is why it is reserved for real grief: a heartrending appeal from a missing child's parents, heartrending scenes at a hospital. Use it for a mildly sad film and it sounds absurd. It is often written with a hyphen as heart-rending, and both are correct. Its close cousin heartbreaking is commoner in speech and slightly gentler in tone.
1adjectiveheartrendingCausing deep distress or grief to witness.
“The letters home make heartrending reading.”
synonyms: heartbreaking, harrowing
From heart + rending.
Etymology from Wiktionary, CC BY-SA