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The number of pixels you need depends on the use you are making of your pictures. If you need to view them on a computer screen, a television screen or simply want them for your web site, you need pictures of fewer pixels. In the same way if you want to make small prints you can use a picture of less pictures but if u want to print a large picture you will need yet more pixels to get smooth looking results.
These pixels are rows of coloured dots that make up a rectangular grid to produce the whole picture. An image on a web page might be made up of 500 rows each with 400 pixels in them. The total number of pixels would be 500x400, which is 200,000 pixels. In digital cameras each pixel is red, green .The pixels can have a number of different brightness values, usually 256 in screen displays, but often 1024 or 4096 in camera images. Because digital camera images are generally larger than this, it gets more convenient to talk about their sizes in terms of 'Megapixels', (Mp), where 1 Mp = 1 million pixels. Our 200,000 pixel image can then be described as 0.2 Mp.
Reducing the no of pixels for making a picture of low resolution is easy, but trying to increase these results in the software having to guess the values of the extra pixels. Different methods of making this guess are called 'interpolation algorithms' and although some make a better stab at it than others, all are basically unreliable. The best method in general use is probably Lanzcos interpolation, found in some cheap software, but inexplicably not currently in Photoshop.
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