Most of my information comes from books, which I can't copy here because of copyright issues. So I ferret around the Internet for accessible references.
The Brown Lady of Raynham Hall is possibly the most famous ghost photo of all time. It was taken at a time when photography consisted of glass plates; one man removed the lens cap, another fired the flash. In other words, a long, long time before Paint Shop Pro and the like. Yes, there are those who have speculated about 'how it could have been done', but this photo has never been proved to have been faked. The photographers even had witnesses present when the plate was developed.
This lady typically appears in the clothing she wears in a portrait in the hall. So, perhaps it's easier to produce an apparition if the spirit has something to work with. If, as I've postulated, a ghost has to create their image before manifesting, then how much easier would it be if there's a big portrait of themselves to work from? Practising creating a copy is far, far easier than starting from scratch.
There are few pictures of me in existence. One of them is of myself at thirteen, on a holiday in Spain. I was rail-thin, dressed in black and wearing a black sombrero (When you're thirteen you think that looks cool. Trust me, looking back on it, I looked like a carpet tack).
Now, if that was the only picture available to me after my death, I would likely model the appearance of any apparition I formed on the image in that photo (I'd probably leave out the sombrero). If I worked from scratch, I might turn out a grotesque parody of myself.
So we have the basis of an experiment: if you know, or think you know, who the ghost is, leave a photo of them around. Let them use that as a template. If they form an apparition, it would be very interesting to note if they appeared as they do in the photo. Then, provide a different photo. This does rely on the cooperation of the spirit concerned, and does provide any deceptive spirit with a potential 'mask'--although even that does not invalidate the experiment.
The one important thing to remember if you do see an apparition is to stay calm. Think about all those people terrified by flickering lights, noises and images of eyeless heads. If someone, or something, has come all the way from the depths of Hell just to turn the lights on and off, present a Halloween face and make 'booga-booga' noises, then it's not scary. If you're in that frame of mind, sometimes it's hard not to laugh.