Decorative images are generally used to make a website more visually attractive, and don’t add any extra information to a page. 

Whether an image is informative or decorative is entirely up to the editor, based on the reasons they have for including the image on the page.

Generally images are decorative when they are identified and described by surrounding text, or when it illustrates adjacent text but does't add any extra information. 

When an image provides no additional information, the Alt text can simply be a blank space between quotation marks.

This is so that the image can be ignored by assistive technologies like screen readers, as having Alt text here would add audible clutter.

Clickable images that supplement link text are also considered decorative.

Image
" "

This photo of a newspaper is used to make the link to the news and events page easier to identify and to increase the clickable area.

However, it doesn’t add to the information already provided in the link text (of the same anchor).

In this case, enter the following in the Alt text section: " " 

 

Images used for ambiance only

Image
campus on an autumn day

This image of the campus is purely used to add visual interest to a page.

Again, the Alt text should read: " "