He did, in classic Sapkowski style, he described the helmets as such right near the end of the final book of the main series. And Cahir's helmet is supposed to be huge.From what I remember, Sapkowski didn't describe there armor as monstrous. I recall Cahir fairly unremarkable plate armor with wings attached to the helm. Nilfgaadian knights are said to have ornamentation on their helms, but otherwise it is fairly ordinary.
I assume he would have used a different illustrator if the armor was weird and monstrous looking.
Not to mention that it looks a lot better.
Authors usually don't get to choose the cover artists for foreign editions, and given that there are Polish editions with video game artwork, I don't think he had much of a say even back home.