Changing the size of an image using image editing apps couldn’t be easier. To change image size, image editing apps come with built in tools that let you quickly change three separate image size values: width, height and resolution.
Image width and height are measured in pixels, inches or centimeters. When you change the width and height of an image, the image editing app automatically adds or takes away pixels to increase or decrease image size.
Adding more pixels may degrade the sharpness of the picture or cause jagged edges to appear. Reducing the number of pixels in an image won’t affect the image quality as much.
For example, in the photo below, the background looks blurry and the picture is somewhat jagged when the image size is increased.
The quality is visibly lower when the image size is increased because the image editing app has to create new pixels to cover a larger area. The app only has access to the pixels in the original image and cannot get fresh new details of the scene already captured in the image. Instead, the image editing app guesses at how to color new pixels to cover a larger area. The jagged edges and burry quality are the results of that guessing.
The process of creating new pixels by adjusting image size is called resampling.
The last component of image size–resolution–determines the fineness of detail in an image and is measured in dots per inch. It doesn’t affect how the image looks on your screen, but it influences the quality of the image when printed. You can read more about image quality for print here.