By changing the image resolution, the number of pixels of information in the image remains constant and, therefore, the file size also remains constant. The second method involves changing the pixel dimensions of the image by resampling. When pixel information is deleted from the image it is said to have been 'down sampled'. When pixel information is added to an image it is 'sampled up'. With a constant image resolution, down sampling produces a smaller image and sampling up a larger one.
Scanned images are often down sampled to create smaller file sizes.
Once an image is down sampled, returning to the original pixel dimension by sampling up can produce image degradation due to interpolation. There are a number of interpolation algorithms used by graphics programs in sampling up images. Three of these are nearest neighbor, bilinear, and bicubic. When you find it necessary to sample up photographic images, try using bicubic, it is the slowest method, but generally the most accurate. For quantitative analysis of graphic images, sampling up by any method is generally unacceptable.