Image resolution is the ratio of pixels or dots in your image to output size. It tells you the number of dots or pixels per inch in your image, and is usually expressed as dpi (dots per inch) for printing purposes and ppi (pixels per inch) for onscreen uses. Generally, image resolution is used for printing, as computer screens usually display images at set scale (72 dpi for Mac or 96 dpi for Windows), although you can zoom in or out of the image. For adequate photo printing, 100 dpi will work, but for high-quality prints, 300 dpi or higher is required.
Open your image in your photo editor.
Check the total height of the image in pixels.
Check the total height of the image in inches.
Divide the total number of pixels for the height by the height of the image in inches. For instance, if the image is 1,000 pixels high and 10 inches high, you get 100. This means the image is 100 ppi, which will translate to 100 dpi when you print it, a fairly low resolution photo.
Increase the sharpness of the image by making the total size (or output size) smaller while keeping the number of pixels the same. If you print the same 1,000 pixel high photo at 3 inches instead of 10 inches high, you get a photo with 333 dpi, a very high quality resolution.