How to create the effect in this animated GIF of morphing faces?

I saw an amazing animated gif

And I thought I should give it a go.

I tried laying out several friends' pictures in movie maker and power director and though I sort of have a video where it is animated up to an extent, it is not as fluid, it feels more like a slideshow no matter how much I tweak the time/tween.

I thought, may be I need to fine control it and opened up Photoshop and added layer mask to go from image to image...though this is a lot better, I still don't have this fluidity.

I am not sure this is the right place to ask this question, but do any of you know how this method can be achieved? I think it's a great pic!