In a previous writing I wrote why eyewitness testimony is almost worthless (you might want to read that article before this one, if you haven't don't already so).
The second most common piece of "evidence" often presented to prove the existence of UFOs and ghosts is photographic evidence.
Photographic evidence can be really alluring. After all, you don't have to trust someone's word or flawed perceptions and memory anymore. Instead, you can see it for yourself. Not only that, it's even better than seeing it in person, because if it was a very brief happenstance, you don't have to rely on remembering everything that happened in that sudden moment, but you can instead examine the photograph for as long as you want.
Also, we are accustomed to the accuracy of photography. When we take a photograph of something and later examine it, the reproduction fidelity is amazing. It's almost always a perfect representation of the original view, at the utmost detail. Because the average person sees hundreds and even thousands of such accurate photograph during a lifetime, we gain an instinctive trust on the accuracy of photography. "An image is worth a thousand words", "a photograph doesn't lie", "pics or it didn't happen" are some of the most common colloquial manifestations of this trust.
And that's the major problem with this: We trust photographs way too much. This trust is so ingrained in us that we often want to believe a photograph even when we know perfectly well that it could have been manipulated or otherwise staged.
Moreover, we trust photography way too much in situations where we are completely and absolutely sure that the photograph has not been manipulated afterwards.
The problem is that photography does not work in the same way as our eyes, and there are often side-effects and artifacts that are produced by the photographic process, which do not correspond to our own perception of reality.
A lens flare is the most common example (although in no way the only one). This happens because it's physically impossible to build a lens that would only let light pass through, without reflecting some of it back. It's a bit like a half-mirror (although the amount of light reflected back is way smaller than the light that passes through). The amount of light reflected by a lens depends on the angle of incidence and strength of the light.
This causes a side-effect, a visual artifact when the camera photographs a strong light: Light reflects from the numerous lenses in the camera back and forth and produces a so-called lens flare. (These lens flares are often round but can also have other shapes due to the internal structure of the camera. A hexagon is a common shape due to how the shutter of the camera is shaped. They can also be of different colors due to diffraction and other reasons.)
Other common artifacts include streaks of light and color bleeding. (Color bleeding happens when too much light hits the photographic film or digital sensor at some point: It overwhelms the spot it hits, and causes adjacent areas to the illuminated as well, causing what looks like a "halo".)
A slightly less common (and consequently less expected and more confusing) artifact is related to the focus of the camera.
The eye focuses at a certain distance in a very similar way as a camera does. However, our brain tends to ignore details that are out of focus and doesn't pay attention to them.
A photograph, however, confuses our brain. Since we are focusing on the photograph and not somewhere else, our brain expects everything on it to be on focus. However, if there was something out-of-focus when the photo was taken, it will look blurry or cause all kinds of other artifacts.
If the out-of-focus object is just slightly blurry, it's easy for us to interpret its meaning. However, when something is way, way out of focus, we are often unable to identify the object because it's way too blurry or distorted.
A bokeh effect is a very typical manifestation of this. It happens when an object or light in the background or foreground is extremely out of focus. It usually causes a big "halo" which might be round or, as with lens flares, have other shapes due to the internal structure of the camera (a hexagonal shape being fairly common due to many camera shutters being hexagonal).
This can be especially confusing in video footage. For example, if there's a small but brightly illuminated object floating close to the camera but completely out-of-focus, it will look like a big floating halo.
More confusingly, it will often look like this big halo moves behind bright in-focus objects in the background. This is due to another artifact in photography closely related to color bleeding: Bright objects tend to "override" faint semi-transparent details (such as a bokeh "halo") and it will look like the bright object is in front of the "halo" when in actuality it isn't. (Basically, it's just "shining through" this halo and overwhelming it, making it look like the halo is behind it.)
Lens flares, bokeh effects and other photographic artifacts can sometimes have more intricate shapes due to the light source having a non-regular shape. A very typical example of this happens when filming a campfire or bonfire, because of the shape of the flames.
There are a myriad of other artifacts besides these that can happen in photography (both while taking the photograph, due to either the physics of light going through lenses or the mechanics of the camera, and while developing it, if talking about photographic film).
Sometimes more than one of these artifacts are in play at the same time, and might blend to look like it's one single phenomenon, when in fact it's just the combination of two or more photographic artifacts.
Naturally pareidolia plays a huge role in all this. (Pareidolia is the natural instinct the human brain has to find recognizable shapes in random patterns. It can be greatly helped and strengthened when the person expects to see something, eg. because someone told them what to look for.)
Of course not all of such phenomena are caused by the camera itself. Sometimes it happens due to atmospheric phenomena (for example, sometimes "halos" in such photographs are not actually caused by photographic artifacts but due to atmospheric effects; of course it becomes even more confusing when both effects happen at the same time in the same photograph).
If you look at "genuine" photographs of "ghosts" and "UFOs" out there, you'll quickly notice that the vast majority of them can be easily attributed to photographic artifacts or atmospheric effects.
And then there are, naturally, the outright hoaxes. Events can be staged and rigged, and photographs can be manipulated. The hoaxer will often maintain his or her innocence and be very convincing at it, but we shouldn't get fooled by such parlor tricks.
For all of these reasons photographic evidence is basically as useless as eyewitness testimony. It can be used as a startpoint for further investigation, but no conclusions should be drawn from it.
Here's a list of things to understand about photography and to look for in photographs of alleged supernatural or otherwise unexplained phenomena: