What the hell is happening on this album cover?

My suggestion is still correct.

If you listen to the album with headphones, you can hear the tree people answering him back.
The tree people actually outnumbered the people inside the Greek audience, there were close to 50,000 of them filling the park surrounding the Greek. When Neil found out earlier in the day that the fire marshal and police were going to clear out the park because it was fire season, he refused to do the concert. He said no tree people, then no concert.
It took a special meeting of the city leaders to override the fire marshall and add more police patrols so the concert could go on. They were afraid that if Neil didn't do the concert the city would break out in riots. And the rest is history.

No lie. Wow, thanks, DiamondDazed. That's a new one on me.