Use 2d Polygon Collider to detect OnMouseEnter on UI

I have a UI image that is a triangle, and while the image is a box of course I need to only detect OnMouseEnter on the triangular area. However I cannot get this to work. I have tried adding a PolygonCollider2D to my Image and set it to isTrigger = true, but I cannot get it to log anything! Any help is appreciated!

I made the collider a bit bigger so you can see it

My code is attached to the Image with the PolygonCollider2D and right now is very simple, but is not logging anything:

 void OnMouseEnter()
 {
     Debug.Log("Collider Entered!");
  }