Use CSS background-image property for decorative images. This means the image

• gives no information at all;
• makes your website more attractive only; and
• is just a background image.

Use the HTML image element (<img>) for meaningful images. This means the image

• needs alternative text;
• must exist to give users information; and
• is not a stock image.

A bonus tip for all of you: reduce the file size of your image. Do not make your users download 10 Megabytes (MB) of image when it can be reduced to 1 MB—or even less.