When Ghalta enters the battlefield, it and Cloudstone Curio trigger.
Resolve the Cloudstone Curio trigger, returning Mirror Image to your hand if its on the battlefield, or returning nothing to your hand otherwise.
Resolve the Ghalta trigger, putting Mirror Image onto the battlefield as a copy of Ghalta from your hand.
When Mirror Image enters the battlefield, it and Cloudstone Curio trigger.
Resolve the Cloudstone Curio trigger, returning Ghalta to your hand from the battlefield.
Resolve the Mirror Image trigger, putting Ghalta onto the battlefield from your hand.
Repeat from step 2.
This clever loop lets you recycle a massive green dinosaur indefinitely by turning it into a game of musical chairs. Because Mirror Gallery is on the battlefield, you don't have to worry about the legend rule destroying your copies, allowing Ghalta, Stampede Tyrant and the cloned Mirror Image to safely trade places. Each time one enters, Cloudstone Curio triggers to bounce the other back to your hand, fueling an endless cycle of creature entrances and leaves.