When you change lenses on an SLR it’s surprisingly easy to get dust on the image sensor that then shows up as dark spots in your images. One way to check if this has happened is to take a few photos of a clear sky or white card. Open the images in your photo-editing program and flip through them. If all of the images have dark spots in the same place, that’s dust on the sensor. To clean the sensor yourself you need sensor swabs, cleaning fluid and courage. NEVER use compressed air, or other cleaning products, on the sensor. Proceed at your own risk. This is a high-risk procedure and it is more prudent to have it done by your camera company’s service center.