Sorry for the delay, @Quantumgo.
1) Both images have to be wrapped with div and rotation transform should be applied on hover state for that div, NOT FOR IMAGE ITSELF

2) The back side of the flip-object should have z-index higher, than front side
3) I would recommend keeping front side image position: relative and only back side image made position: absolute. It will help to keep wrap div its height.
4) "Parent div" should have to have set Perspective, for make visual 3D effect for flip animation

Hope I was able to explain.
Cheers,
Anna
P.S. Don't forget to find identical phone prototype images. Now there are 2 different models and it looks weird.