The image size (or pixel dimensions) of an image is a measure of the number of pixels along an image’s width and height. For example, your digital camera may take a photo that is 1500 pixels wide and 1000 pixels high. These two measurements indicate the amount of image data in a photo and determine the file size.
Resolution is the amount of image data in a given space. It is measured in pixels per inch (ppi). The more pixels per inch, the greater the resolution. Generally, the higher the resolution of your image, the better the printed image quality. Resolution determines the fineness of detail you can see in an image.
Although a digital image contains a specific amount of image data, it doesn’t have a specific physical output size or resolution. As you change the resolution of an image, its physical dimensions change, and as you change the width or height of an image, its resolution changes.