The dots per inch (DPI) of an image is a measure of its resolution. The higher the DPI, the sharper the image will appear. You can check the DPI of an image on a Mac using the Preview app.
To check the DPI of an image in Preview, open the image and then click on the “Tools” menu. Select “Show Inspector” from the menu. In the Inspector window, click on the “Info” tab. The DPI of the image will be displayed in the “Resolution” section.