Photo was shot with a Canon EOS Rebel T1i using a Canon EF-S 18-200mm f3.5-56 IS lens.... Lens was zoomed all the way to 200mm and was wide open at f5.6 and shutter speed of 1/1600 sec.... ;)
What you seen in the photo is probably vignetting:
Normally you really only notice this in the corners and quite such a big ring.... However check this page:
Looks like you can reduce the vignetting quite a bit by simply stopping down the lens a bit (they stopped down to f8.0).
Bottom line is, what you see in the photo is "normal" for this lens... if it bothers you need to understand how to avoid it when taking the photo and/or how to correct in your post processing...
hope this helps... ;)