What are broken CSS images?
Style sheets often contain links to images. When frequent changes to the website and style sheet take place, it's possible that the references to the images have expired. Although the images may not be used anymore, the links to the deleted images are still present in the style sheet. If web browsers attempt to load the deleted images, there's a good chance that this will cause problems or disruptions in the user experience.
When this happens, the visitor of the website only gets to see red crosses where the images should have been. These are called broken images. The broken images can also be responsible for contamination of the 404 not found logs of the web server.