sight gag

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A comic bit or effect that depends on sight rather than words.

Wiktionary

1. n. film, television A visually presented action or device which causes a humorous response without requiring any accompanying commentary or other speech.

WordNet 3.0

1. n. a joke whose effect is achieved by visual means rather than by speech (as in a movie)