Hi Falvio- I think this is an Elegant Tern, Sterna elegans. Note the fine, slightly down-curved bill (Royal's is heavier straighter) and the orange with paler tip colouration (Royal's is even orange-red). Royals are almost twice as heavy as Elegant Terns and I think you can pick up this "daintiness" from your image.
Agree, and the rather wide black on the side of the head and the shaggy crest at the back of the head would tend to support Elegent as against Royal. A few years ago we had a chance to observe a nice mixed colony near La Jolla, and had an opportunity to compare the two. Some of them were very hard, and I think it's an under-appreciated and subtle ID problem. I like the sharpness and pose of the image, although I would have preferred removal of the shadow of the bill on the wing.