Lead-ins (aka "kickers") are used to catch the reader's attention and "lead in" to the main caption. These phrases widely range from common phrases to song lyrics, and are written appropriate to the subject matter of the photograph.
Examples of lead-ins include the following:
Editors and caption writers tend to avoid using clichés and obvious lead-ins (e.g., "Two heads better than one," "Whistle while you work," etc.); and also prefer varying lead-ins throughout the publication.