Can I clean the inside of the cam?

Hope this is the right forum, been taking photos on my XTi and I noticed a darker smudge in the corner of photos with skies. Its not really visible in other photos and I tried different lenses so it's the camera. http://i4.photobucket.​com …/Vivieri/asfnnk​streak.jpg

Someone suggested mirror-locking and checking for hairs and, ahoy, a cat hair in the back! Got that out and haven't been able to check if that was the problem BUT! I did notice a white mark here http://i4.photobucket.​com …y137/Vivieri/IM​G_9501.jpg (upperleft of the light reflection)...dried water? dust? a scratch? Can I use anything to try and get it out?

Nothing on the mirror or the focusing screen can affect an image (except for possibly buggering up the autofocus or auto exposure systems).

What you see in your image is either from some stuff deposited on the image sensor (actually a glass anti-aliasing filter in front of the sensor) or from a damaged shutter.

That pic you linked, the second, is of a shot of the mirror. This has no affect on your image. Use the sensor cleaning feature and take a look at the sensor (in a dust free place of course). If you see anything, try to blow it out with a Rocket Blower.

I'm guessing that a hair has fallen across the sensor. You can clean it quite easily with a bulb blower. Put the camera in manual cleaning more with the lens removed. Hold the camera lens opening down and give it a few aggressive puffs from the blower. If it's a hair, it should come right out of there. You might also take a peek in there as well.

You might try this as well, it will make the hair more clear. As the pic you took was at f/5.6, the sensor blemish is blurry. To make it more clear try the following.
1. Use any lens and set the camera to aperture priority.
2. set the lens ar a minimum of f/22 but you can go smaller if possible.
3. unfocus the lens as much as possible and take a picture of a blue sky. You can even introduce some motion blur as well as all you want in focus is the crud on the sensor.
4. Bring the pic into photoshop and use the "Auto Level" function on it. The hair should be much clearer as will other sensor specks.