Asalamualaikum wrt wb,
I just found two important resources for you. The first is the US State Dept. guidelines regarding wearing religious attire for the Passport Picture. It states on the US govt website:http://travel.state.gov/passport/guide/faq/faq_881.html
Can hats or religious headgear be worn for the photo?
Unless worn daily for religious reasons, all hats or headgear should be removed for the photo. A signed statement from the applicant must be submitted with the application verifying the item is worn daily for religious reasons. In all cases, no item or attire should cover or otherwise obscure any part of the face.
The second is the case of a Sikh who was discriminated against and not allowed to take his passport picture because of his turban:http://religionblog.dallasnews.com/archives/2009/03/postal-service-apologizes-to-s.html
My suggestion for you is to request a meeting with a higher up in the embassy, since many times the lower workers are local and do not know all the regulations.
You can print the previous two documents and explain (to the US consular, not the workers) that the hijab is part of your religious uniform that you wear daily.
If you still have problems, please write back, and inshallah I will consult the scholars.
And Allah knows best.