Listed below are some excellent sites for finding images in the public domain
Dance Online: Dance in Video
Five hundred hours of dance productions and documentaries by the most influential performers and companies of the 20th century. Selections cover ballet, tap, jazz, contemporary, experimental, and improvisational dance, as well as forerunners of the forms and the pioneers of modern concert dance.
Digital Theatre Plus
Streaming video of full plays, musicals, and operas from contemporary British companies, plus educational materials about the plays and their creative and technical production.
Electronic Arts Intermix Educational Streaming
More than 500 full-length videos from EAI's extensive catalog of video, media, and performance art from the 1960s to the present.
Over 1600 videos of opera, ballet, and classical music concerts, along with documentaries and artist profiles. Search and browse by artist, composer, director, etc.
Met Opera on Demand
More than 450 recordings of Metropolitan Opera productions, in streaming HD, video, or audio, dating from the 1930s to the present.
Music and Dance Online
Searches across the following Alexander Street Press databases (some are individually linked above):
Naxos Video Library
Over 1580 full-length videos of classical music performances, opera, ballet, live concerts and documentaries. Search and browse genres, composers, labels, artists, and production personnel.
Streaming HD video of contemporary dance, theater, and music performances from Seattle-based company On the Boards.
Opera in Video (Alexander Street Press)
Five hundred hours of streaming video of the most important opera performances: staged productions, interviews, and documentaries. Selections represent the world’s best performers, conductors, and opera houses and are based on a work’s importance to the operatic canon.
Theatre Online: Theatre in Video
More than 250 definitive performances of the world's leading plays, together with more than 100 film documentaries, totaling more than 500 hours of streaming video representing hundreds of leading playwrights, actors and directors.