Here’s a very useful page from New York Times: 42 curated pictures from their “What’s going on in the picture” section. The pictures have no captions or explanations, so are a perfect tool for building hypotheses and discussions. Then you can click on the link and find out more, to have the students check their ideas; there are sometimes comments by other students, as well as more discussion tasks. An excellent resource with built-in feedback!
P.S. And if you prefer Dixit-style picture prompts, NY Times has it all. This article has a great collection of prompts, lesson ideas and absolutely amazing moving pictures – all kid-friendly, mind you.