Remembering the definition of “sexy”, sexy photos cause arousal and imply the (fantasy) that sex is possible or maybe even imminent. This means that, most times, the subject needs to have a connection with the viewer. The easiest way for this is for the subject to be looking into the camera, so there is eye contact with the viewer. For example, here is a photo of the lovely Nikkii:
However, the arousal can also come without eye contact, if the rest of the photo sets a mood or scene where sex might be occurring soon. For example, in this photo of Chelsea Christian, she is pulling on her top in a way that will expose more skin.
This connection explains why some nudes are non-sexy or far less sexy than photos showing much less skin, even though the nudes show what is not normally seen. Examples include most bodyscapes, many art models for figure studies and the resulting fine-art nudes, and ethnographic photography.