Marks or blemishes appear on all my photos

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Minor: Look at the lens--most likely you'll find a speck of dirt or a smudge on the glass. Use a clean, lint-free cloth (available at most electronics stores) to brush away any dust particles and to remove smudges. Avoid using a shirt or cloth that could leave more particles behind or that might scratch the lens.
 
Moderate: If cleaning the lens doesn't do the trick, the obstruction may be on the CCD sensor that acts as solid-state "film" for digital cameras. To get a sense of the problem, take a reference shot of a white sheet, a cloudy sky, or a blank piece of paper; dust specks really stand out in a white photo. To clear debris, digital single-lens reflex users can remove the lens and spray a shot of compressed air (sold in cans) into the space behind it. Owners of other models can try extending the camera lens and shooting air at the seams. Just make sure that you do this carefully in a clean, dust-free environment.
 
Dire: To remove stuff that won't blow off, SLR owners can try using methanol and a lint-free cloth (attached to something like a cotton swab) to gently mop the CCD surface. This procedure can be tricky, so you might opt to have your camera professionally serviced instead. If that doesn't do the trick, you probably have a scratched lens or a damaged CCD. Owners of fixed-lens digital cameras must seek professional service, too.