Cutline

A cutline is the caption (text) near a photograph in a newspaper. It informs the reader of who, what, when, where, and why or how about the photograph. Because photographs depict events frozen in time, the first sentence of a cutline is written in the present tense. Additional sentences can be written in present or past tense depending on the publication’s style preferences.

Also called an underline, legend or caption.
See additional information at "How to write a cutline."