Tiffani Thiessen -- I'm DROPPING Instagram Over 'Ridiculous' New Terms

The Instagram mutiny is underway ... with a bunch of celebs already vowing to NEVER use the photo-sharing service again after the company announced a HUGE change in policy.

Instagram has been under fire for updating their terms of use ... which says users "agree that a business or other entity may pay us to display your username, likeness, photos (along with any associated metadata), and/or actions you take, in connection with paid or sponsored content or promotions, without any compensation to you."

The statement pissed off tons of users -- famous and non-famous -- including Kate Walsh and Tiffani Thiessen .. who tweeted, "Really sad to have to end my luv 4 @instagram. Will be deleting my account due 2 their ridiculous new terms."

And there's more ... "Dancing with the Stars" pro Cheryl Burke is also abandoning ship ... tweeting, "Can't believe Instagrams new policy! They r allowed 2 use ALL of ur images in advertising. Will be erasing my account shortly."