Here's a better image of the ardashir relief:
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The above image is called the "investiture of ardashir". Basically the Sassanids used to incorporate Zoroastrian religious concepts very tightly with the imperial state. So all the Shahs used to consider themselves divinely mandated by ahura mazda and the prophet mithras or zoroaster. This idea was directly imported into Islam (medieval islamic kingdoms were pretty much based on persian ideals) so instead of Ahura Mazda and Mithras, the sultan would seek an "investiture" (ie official right to rule) from the Caliph who was the viceroy of Allah on earth.
Basically speaking, Zoroastrianism and it's relation to imperial Persia directly influenced medieval Islam.
EDIT:
Also if you look at the bottom of the stone relief you'll notice dead bodies under the foot of the Shah and Ahura Mazda. Even Zoroastrianism associated their religion with the vanquishing of enemies of both the Shah and of God. Islam did not invent religious warfare.