Sprite transition?

I've the beginning of the image and the end result of the image (glow intensified). When I animate the 2 images it just flickers back and forth. How can I smoothly transition to the next image? Maybe fade from 1 sprite to the other? Do I need more images for the animation?

 

select the animation, and in the inspector un-check loop time

 

It won't loop then. I want it to loop, but I want to to fade from 1 sprite to another.