How to make a slime with a custom head?

I am trying to make a slime look like a frog (using custom skull texture):
Frog:
Display: '&6Frog'
Type: SLIME
Health: 10
Options:
Size: 1
Angry: false
Despawn: true
Silent: true
DamageModifiers:
- PROJECTILE 0.2
Disguise:
Type: player
Player: Frog
Skin: Notch
Invisible: true
Equipment:
- FrogHead:4

But the mob is only invisible and it does not work as expected. Any idea?