When a digital image is enlarged or reduced, its pixel dimensions are changed. The image needs to be resampled to bring out detail.
Photo editing programs resample images automatically. Some programs provide the option to turn it off or choose the type of resampling.
When resampling an image that has been reduced in size, the number of pixels decreases. Pixel information is actually deleted from the image.
When enlarging an image, the number of pixels is increased. New pixel information is added based on matching color values of existing pixels within the image.
As a general rule, don’t resize images more than once or, in the case of an image reduced in size, twice. Resampling too much causes images to lose sharpness and detail.