The place where in general you explain them the rules, and they get the job done: Casa Fa, Av Pueyrredon 1033, corner Av Cordoba.
They were just bought by Chinese (I think, definitely Asian) very very nice people, the pics they gave me the first day got dirty from the ink from their stamp. I called them on the phone and spoke to their Argentine employee who said the pics had already been deleted and I'd need to take new ones. I came in, spoke to the Chinese lady who I think is some sort of owner, she found me yesterday's pics and refused to take more than 10 pesos for new prints.
Not sure if they'll email you their file, ask them on the phone beforehand. They have a dedicated booth and shouldn't have issues with shadows.