Hi Shirley Ann, the problem will almost certainly NOT be connected with the lens. Any marks on the lens would also not have any effect on the image, as they would not be brought into focus on the sensor. They would simply reduce the effective aperture of the lens and hence the amount of light passing through to the sensor. Your camera instruction book should tell you how to raise the mirror to clean/examine the sensor. If you do this you should be able to see any large dust or smears on the sensor.
Alternatively, fit a wide-ish angle lens on your camera, set ISO to 100, then using a long exposure time and a small aperture say, f16 or f22 take a shot of a white ceiling whilst moving the camera around throughout the exposure. Then examine the shot on the camera screen by scrolling around it at a high zoom level. If your sensor is clean you should just have a blank white/greyish image with nothing in focus. If there is any dust on the sensor it should show up as darkish spots or marks at a high zoom level.
I think that it is inevitable that all sensors are going to need cleaning from time to time either from dust entering the camera during lens changing or even lubricant from the shutter mechanism getting onto the sensor. Therefore I have invested in an illuminated loupe, an Artic butterfly brush, a Lenspen Sensorclean pack, as well as wet cleaning equipment, although I have not yet needed to use the latter two to date. Sometimes simply using, say, a Giottos blower can be enough to dislodge any dust without needing any further attention.
I hope this is of some help. Please feel free to PM me if you need any more explanation
