Problems with the CSS background-image property
The article criticizes the abuse of background-image as a cheapass replacement for the img or picture tags that should be used for "content images".It's not a controversial point of view: background-image has always been intended to be purely ornamental (for cases in which background-color doesn't look good enough) and there's no reason to worry about "semantic" features like alt text (a blind or text-only user doesn't care for how the page background is painted).What perverted line of thought led to the apparently widespread abuse of background-image for content? 
CSS sprites, as suggested in other comments, are a significant hack that might have set an example but true content (rather than icons) in background-image is on a different level and requires additional explanation.