First, as always, let me start by saying that I do not recommend still photography for paranormal investigations. Still photos will almost never stand up to scientific scrutiny.
Having said that, if you want to use your still camera for investigating, ALWAYS make sure your camera is clean inside and out first before your investigation begins.
We see many photos presented as evidence that are really the result of a dirty camera, either a dirty lens, or dirt on the camera’s sensor.
If you notice a dark, shadowy spot or black spec that is in many photos, that does not change position from photo to photo, it has to be something from your camera.
Notice the photos attached. The photo showing a piece of paper is an example of dirt on the lens, and the other photo of the sky is an example of dirt on the sensor. The dirty sensor photo is an extreme example, and you will probably never have your sensor this dirty, but a few specs are not uncommon.
Make it a habit to clean your lens every time you take it out, even if you just cleaned it the last time you had it out. Dirt, dust and finger prints accumulate rapidly. Dirt and dust can accumulate on your lens even from inside your case. Any where there is atmosphere, there is dust.
Check you camera’s sensor for dirt regularly by taking test photos against a plain background, such as a clear sky, a plain piece of paper, etc. Make sure you clean your lens first to ensure that any dirt spots are on sensor, not on the lens.
If you find that you do have a dirty sensor, be very careful in how you go about cleaning it. Check your camera’s manual and follow the directions CAREFULLY. Do not ever spray anything or stick anything, such as a cloth or q-tip, into your camera unless you are instructed to do so by the owner’s manual. Some cameras have onboard automatic sensor cleaning, but they aren’t perfect. If you are not comfortable cleaning your camera’s sensor yourself, you can have it professionally cleaned at most camera stores.
* Dirty camera sensor photo courtesy from vogeltalksrving.com