Why is there a black dot on all my photos?

If you have another lens, go ahead and try with that to see if the issue comes from the lens or camera sensor. If the spot remains then its sounds like it could be some dust or debris on the sensor itself. The way I go about cleaning DSLR sensors, I remove the lens leaving the cap off, at this point you should be able to see the camera mirror, now get yourself one of those hand air pumps that you squeeze and blows air out and what you're going to have to do is set the camera to 30 second shutter or bulb if you have that option. The goal here is to take a 30second or longer exposure in which the camera shutter is going to open and reveal the sensor and you need to gently blow air onto it to clear off any debris, do NOT use anything else other than a hand air pump to clear dust as you could make the situation worse. To avoid external dust getting inside while the shutter is open for those 30 seconds, I usually hold the camera upside down, making it harder for dust to settle on the sensor and I also like to turn on the shower on hot for a while to steam up the room which allows dust to settle from the air.

@fredo24477 without actually seeing this "black dot" and only going by your description, this sounds like its time to clean the CCD sensor. (Of course, there is always the chance of a sensor failure).You could get a DSLR cleaning kit and use this Nikon D50service manual. Look around page 60 for the CCD and how to get to it etc. Also, check this site about the self cleaning function on your D50