I want play sound when 2 image are touching each other. Sorry for my bad english. 
Attach a box collider to them and make sure they have a Rigidbody2D component attached as well. Check if the colliders have correct dimensions too, or else collisions may fire incorrectly.
However, UI objects aren’t really for this kind of thing. I don’t know your situation, but if you’re looking for run-of-the-mill type UI, you might want to use sprites instead.
Take care.