Four Basics
Aspect ratio, image size, file size, and resolution all pertain to picture and graphics sizing, and here’s a quick primer on what they are and why they matter.
Asking “what is the standard image size for a website?” is a little like asking “what’s a standard font size?”. While there’s a general range we stay within, and there are preferred aspect ratios for specific platforms, the variety is endless, so it’s important to size each image to its intended use.
When choosing the best image size for your website, we will examine where and how we’re using the image. Will it need to be different sizes on desktop, tablet, and mobile, or will it appear the same size on all screens?
For an introduction to image formats (.jpg, .png, .svg, etc.) see my library post here.
Aspect Ratio
The aspect ratio of an image is the relation of its height to its width, written as height-x-width or height:width. For example, 2×3 or 2:3. In photography, the standard is to list height first. You can change an images’ aspect ratio by cropping or by stretching it.

Below is the same image at 100% aspect ratio, cropped, and stretched.



Image Size
An image’s size is its actual dimensions in inches, cm, or pixels. Printed pictures are often 2×4 or 3×5, but we’re unlimited in our options online.

File Size
The file size is how many kb, mb, or gb the file is. Below is a screenshot of these files in their folder. Look at their file sizes in the right-hand column.

Resolution
Resolution is the level of detail an image has. In digital terms, that means how many pixels it has. The more pixels, the higher the resolution. Zoom in on these two images and you’ll see the difference in clarity. The left one is 72ppi and the right one is 22ppi. Image resolution can be increased; I use Adobe Photoshop for that purpose.


Image sizing apps
Images can be resized in a number of ways, some of which affect the quality and some of which don’t. I used Adobe Suite products to resize my images, but that involves a monthly subscription, which many people don’t really need. Below are two free online image resizers.