How to solve this issue in the photographs?
I have a Nikon D200 camera and when I take pictures, there's always black spots at the top right and left-hand corners. Why do you think that this is happening, and how can i solve this?
Thank you
Example: 
Added (1). Thank you Mathew for the answer.
That's called vignetting. It happens when you camera sees the edges of the lens so to speak. It happens with wide angle lenses. If you have photoshop you can go up to filter then lens correction to get rid of it. Zooming in with your camera won't show it nearly as much also. Otherwise you can crop the image.
Actually, that's *not* vignetting. That's an obstruction of some sort -- either a filter you have on the lens (you don't need a "protective" UV filter) or a lens hood. If you don't have either on the lens, then the lens elements have slipped out of alignment, and need to be repaired.
Like Matthew said, Photoshop's a good way to remove it. If you are using screw-on filters on your lens (for example a polariser or UV filter) it might help if you buy the slimmer (but more expensive) kind.
I personally prefer not to use Photoshop for removing vignetting though, because you have to go into each and every photo to correct it. My method of choice is Adobe Lightroom, where under the 'Lens Corrections' section can click 'Enable Profile Corrections'. This almost always fixes the issue with one click and at the same time removes most of the barrel distortion that comes with using wider lenses. But the best thing is that you can select multiple (or all) photos in an album and apply the lens correction to all of them at once.
Other than that, Lightroom is a brilliant program for archiving and editing photos and definitely worth checking out.
Hope that helped a bit!
Like Mathew said it's vignetting and you can get rid of it in photoshop, gimp or whatever your editor of choice is (as long as it's a good program) it can also be caused by lens hoods and screw on filters though so if you have any of these attached try removing them to see what happens.
It has been caused by an obstruction of some kind.
A few things can cause it
Perhaps you have the wrong lens hood for the lens.
You may have a filter on the lens which is too thick.
Also really cheap wide angle converters can cause this kind of thing.