Google can be very picky about the type of photo they show in their authorship rich snippets. They want the photo to be recognizable and they have many more guidelines.

That being said, one female muslim blogger was upset that her photo was not showing up in the search results as a rich snippet authorship. She asked if it had to do with her wearing a headscarf, since that might make the photo of her face less recognizable.

Google’s Zineb Ait Bahajji Webmaster Trends Analyst said in the thread:

Want proof? Search for [Amina Wadud], a prominent Muslim woman. You will see her image show up, even though she is wearing a headscarf:

Forum discussion at Google Webmaster Help.