A good place to start, is to understand that digital cameras capture images as pixels. "Pixel" stands for "picture element." The pixel is the smallest element in a computer image rather like the tiles in a mosaic that combine to make up a picture. Pixel dimensions are typically expressed as the number of pixels (or dots of information) in the width of the image by the number of pixels in the height of the image. When you multiply these two dimensions together, you get the overall size of the image expressed in Megapixels. Digital cameras are often specified by the size of image they can capture in Megapixels - for example a 2.1 Megapixel camera captures an image at 1600 x 1200 pixels.

25 x 25 pixels (enlargement) 200 x 200 pixels

Now that you understand what the "Megapixel" thing is all about, why is bigger better? As you can see from the pictures above, the more pixels you have the more "detail" you have. You may have heard people talk about "high res" or low "res" images. Resolution is expressed as Dots Per Inch (dpi) when printed, or Pixels Per Inch (ppi) when viewed. The dpi measure indicates the quality of the image, but depends on the required output size. The higher the dpi value is, the higher the density of visual information that is available to represent the image. The more visual information there is the sharper and more colour rich the image.
If the image is enlarged, the dpi value decreases; conversely, if the image is reduced in size, the dpi value or resolution increases. In simplified terms, just remember that the lower the resolution, the fuzzier the image quality appears. The higher the resolution, the sharper the image quality appears.
For example, if an image is 1280 x 960 pixels, and you want to print an image that is 200 dpi, then divide 1280 and 960 each by 200 dpi to get the maximum size that the image can be printed without losing clarity, in this case, 6.4 inches by 4.8 inches. Check our photo quality guide to see how big you can print your photos. You cannot directly compare pixel quality printed by an inkjet or laser with that of a true photographic process.
For the Frogprints photographic print process, the optimum resolution is 300dpi, anything greater than this will have no added benefit. 200dpi is our recommnded resolution for those trying to save space on their computers, as there will be very little visible difference between 200 and 300 dpi. See our information on scanning for a more complete discussion on resolution.