They are often dark spirits, but they can be angelic as well. This is because the angelics are often in close proximity to their humans, as guardians, and must maintain a suitable energy for the role. And this brings them more into the range of the humans themselves, than the light being realm. In this way, they are not so different from the spirit meddlers who enjoy similar energies, although of a lesser degree with respect to intensity, but not necessarily of vibration. So there can be quite a lot of confusion about origin of such images, and it is not easy to assign the interpretation to one or the other.
Human spirits will more likely have a more human appearance because when they become visible, either to the eye or especially in photographs, it will be because they are projecting an image of themselves via their thoughts, their consciousness, that in many ways resembles their form while they were in the living, and so will look quite humanlike. The angelics are light beings, and are pure energy, and only manifest a form like a physical body when there is a need to be understood on the part of an observer. So, in that case, the angelics will put on a show, and will appear as humanlike so the meaning of their appearance will be accepted and understood without great fear, in most cases. This will always be for special circumstances that may include the answer to a passionate request to have an angel visitation, and as a reward for steadfast faith and devotion to a divine path.