Most digital images, whether RAW or jpeg, will need to be resized for either net viewing or printing. This interesting article on Pixiq takes a quick look at the various methods of interpolation available through post processing software. The process of adding or removing pixels can dramatically affect the final outcome of the image.
John Neel writes:
Any image file can be resized to another dimension. To understand what happens when you alter the size of a file, you need to look at the way an image is interpolated. Before I get too far into this discussion, I should say that it is always best to capture the image as large as possible and as close as possible to the size you want to print. I almost always shoot at the highest resolution possible so that I can avoid any unnecessary interpolation up or down. The reason for doing so should be obvious once you start to understand how interpolation, or resizing of a digital file, works.