Circular in/out effect for scene

I want to create effect something like following game has
Spider Monkey.

On scene change there is effect of black circle in and out. For this purpose I have image like as following


Using above image I am scaling in and out but it creates stretched image at circle curve not smooth as above game has. I think, I was not understanding the concept clearly.
Please give some hint in this.

use this shader and use circular gradient image as Slice Guide texture in material. Now change the value of SliceAmount of material and you will get your effect.