from training.npr.org: https://training.npr.org/2016/01/12/these-are-nprs-photo-caption-guidelines/
Captions are journalism, too. They should be fact-checked and typo-checked. They should be complete sentences that present the who, what, where, when and (sometimes) why without necessarily stating the obvious (i.e., he sits, she waves, they clap). Captions give photos context, telling viewers what’s going on in a photo so they don’t have to guess
These tips and tricks will you help write stellar news and feature headlines.