Common questions regarding dust in lenses are, “will the dust show up on my images?†and “should I have the dust cleaned out of my lens?†The short answer is no.
Most lenses have some degree of dust, especially zoom lenses. When you zoom in and out, air is pushed and pulled with dust right along with it. So why do the dust particles not show up on the image? It’s all about physics; the light actually “bends†around the dust particles. If you had a very large particle or conglomeration of particles on the other hand, that would affect your images. Even if by some chance that happened it wouldn’t appear as a spot, but as a slightly dark area of diminished contrast. In some very rare cases smaller dust particles will show up on images; this usually occurs with compact (point and shoot) cameras where the rear lens element placement is considerably close to the image sensor. In those cases it probably would only manifest in small “sweet spots†as you zoom, moving those elements close to the sensor.
While minor dust is not an issue and generally does not require cleaning, sand and other gritty substances are much more harmful; they can seriously inhibit the mechanical functions of a lens. A single grain of sand can actually stop an auto focus mechanism in its tracks and render it completely useless. Sand in the zoom mechanism can permanently destroy a lenses ability to perform smoothly, even after the sand has been extracted because of the friction and ware it was subjected to. Needless to say, extreme care should be used when taking a trip to the beach.
The likely cause of spots on images is dust on the cameras image sensor. While you can clean it yourself on a DSLR body it is highly recommended you have a professional do it for you. The image sensor is the most expensive part of your camera; they are very sensitive and can be easily scratched or damaged.