odd
can you post a photo as an example?
it'll be nothing to do with the mirror as this has no effect on the final image all this does is bounce the light so that you can see thru your viewfinder!

could possibly be something on your sensor, ie dust, which would mean you'd have to send it away to get cleaned as doing a DIY cleaning on your sensor isnt recommended!!

anyway post up an image with the smudge
and ill take a guess!

:]
 

If it isn't on the lens, it'll be on the sensor. If you are just starting out and don't know how to clean it, take it somewhere. The manual will tell you how to clean it, but that decision is up to you. I would advise getting friendly with your local camera shop, or experienced photographer, and asking for help to clean it and show you how to do it yourself. If they will explain and show you as they clean it, whatever their going rate is, it'll be a good investment on your part.
 

it is probably jsut dust on youir sensor. Go out and buy an air blaster, its a little rubber ball that when its squeezed air comes out a little tube in the end. its used to blow off the loose dust on the sensor. if u dont have enough money to send your camera in to get cleaned, just buy a sensor cleaning kit. people say that you shouldnt do it on your own, but i have been doing it for about 4 years now and i will never send a camera in to get cleaned. its cheap and easy. just do some research online
 

You have a $700 + camera, the CMOS sensor is its brain and **EXTREMELY** sensitive. I would NOT recommend spraying anything into it, a lot of the dust sprays have additives and are not just air.

The XTi has a automatic sensor cleaning mode (as well as a manual one). Take a "before" picture of a light colored surface with the aperture wide open. Do the automatic cleaning. After the cleaning, take an identical picture w/ same aperture setting. If you still see the smudge, try the auto clean again. If after three attempts, it is still there, you may have to clean manually.

For this, try your local camera shop first. If they don't do it, get a static brush, they sell those on ebay. I can get you a link if you need it.
Do the cleaning in the bathroom, or somewhere with very little dust, hair and air movement, to prevent more debris from entering the camera. You will need to be VERY CAREFUL and GENTLE when "brushing" the sensor.

Always keep lens caps and camera body caps on and wipe the lens contact end with a clean dust cloth before attaching it into the camera.
 

Lisa, Anytime you change lenses on a DSLR you are prone to getting dust particles into the body and on your sensor. There are some tricks to changing your lenses to avoid this but most of all limiting your lens changes will help when changing also keep yourself in a dust free enviroment. The camera forms a charge internally which attracts air born particles also. Since you have artifacts on your photos most likely you have dust on your sensor. Best you take it to any camera/photo shop who have moisture proof high pressure air compressor and have them clean it properly. My dealer does it from free since I bought my current camera there. Ritz Camera
Strange you should see the dust on dark colors, usually the artifacts show up most on photos of the sky, light colors.
Failure to clean it or do so properly can end up damaging your sensor in the long run. Periodic cleaning is also highly recommended.