Coachella -- Strict Warning to Staff ... NO PHOTOS!!!

Coachella Valley Music and Arts Festival organizers have instituted a social media blackout on its staff ... banning the posting of any photos before the festival's opening day.

According to our hippy spies, Goldenvoice fired off a memo to staff, vendors and artists ... ordering them to refrain from taking photos of the grounds, so they wouldn't "spoil the surprises." What is this, "Game of Thrones"?

The directive, which has been plastered everywhere, implores everyone to keep pics off of social media as well as "PERSONAL, VENDOR, and ARTIST websites."

Once the festival kicks off on April 10, the ban is lifted.

The ban was triggered by a leaked photo on Reddit this week, which appeared to show the festival's main installation under construction (each year there's one signature art piece).

The photo was up for a day or two before mysteriously disappearing ...