The MegaPixel rating for a camera is a rough measure of its image
detail, which is one contributer to overall image quality.
Image quality is also affected by the lens, the type of image
sensor used, the scene lighting, and the familiarity of the
photographer with the camera. But it is important when
viewing and printing pictures, that there be enough detail to fill the
viewing or printing area. Most digital pictures are taken at
a 4:3 aspect ratio (ratio of width and height of the picture), which is
the aspect ratio of most traditional computer and television monitors.
Hence, most digital pictures can be exactly scaled to fill a
computer monitor. Wide-screen monitors and HDTV have a 16:9 aspect
ratio instead. Few prints are printed at either of those
aspect ratios.
The available image detail also determines the largest size print you
can obtain from your image. The following chart, adapted from
the help screens at ClubPhoto
and then augmented, shows the available size prints they offer, and
also the minimum resolution they recommend. I added the Min MegaPixels
column to their chart, which shows the minimum MegaPixel rating for the
camera to produce their resolution, before being cropped to the aspect
ratio of the print, and which is derived exactly from their Min
Resolution column. I'm not sure that all their resolutions
are consistently related to the Print Size, and that may have something
to do with the type of equipment they have, their expectations of
viewing distance for different size products, and/or their opinions of
customer satisfaction with different size products.
Personally, I'd recommend about twice their minimum resolutions.
Note that twice the resolution would
become 4 times the minimum MegaPixels, becausewidth and
height get multiplied together to obtain MegaPixels. Even
more MegaPixels are better yet, because that allows the image to be
cropped in all directions before being printed, and still have enough
image detail remaining to produce a decent print.
It is good to note that if you want complete control over what gets
printed, you should pre-crop your pictures to the exact aspect ratio
you wish to print, and leave a bit of border around the desired image,
to allow the print vendor to crop just a little in all directions to be
sure they leave no white edges on the picture itself, and to allow for
a bit of obscuring of the edge of the print when matting or framing.