This is a still from the set of a Muppets TV special and I'm pretty sure it's the 1979 The Muppets Go Hollywood, which celebrated/promoted the release of The Muppet Movie. It's interesting because you can see the proximity of the Muppeteer (Frank Oz) and the human in the shot (Dick Van Dyke). One of the many innovations that Jim Henson either invented or perfected was having his characters perform right alongside people instead of having the puppet in some kind of stage separated from any non-puppets in the scene.
This photo is either from a rehearsal or a number in which Mr. Van Dyke and Ms. Piggy were lip-syncing to a pre-recorded track. How do we know this? Because Frank Oz isn't wearing a microphone, that's how we know this.