What are examples of stimuli that we know cause a grief or sadness reaction in humans? I am looking for examples of stimuli that has been shown to cause sadness in humans in a controlled environment as opposed to the neurological basis or evolutionary reason for sadness
I m not sure how much of sadness images are covered under it but perhaps you could have a look at IAPS ( International Affective Picture Series) . They seek to make media for specifically inducing desired affective states in research work including images , audios and videos.

http://csea.phhp.ufl.edu/media.html

Perhaps you could have a look into Mood Inducing Procedures ( MIP) used to induce both positive and negative affects in a laboratory setting .