The ratio of the width to the height of a screen or an image is called aspect ratio. The aspect ratio of an image describes the proportional relationship between its width and its height expressed as two numbers separated by a colon, as in 16:9. For example, an image with an aspect ratio of 16:9 might be 16 inches wide and 9 inches high, or vice versa. You can use pixel size calculator to know the pixel size of the image and aspect ratio calculator to know the exact aspect ratio of your image.
Learn more about Aspect Ratio:
Aspect Ratio: What it is and Why it Matters by Digital Photography School
How to Maintain Image Aspect Ratios in Responsive Web design by Sitepoint
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