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

34021-faderlarge.png

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 http://wiki.unity3d.com/index.php?title=Dissolve_With_Texture 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.