Some important reiteration on how Google relies on HTML tagging and ALT tags, to derive page information and how this drives rankings.
Why Add Alt Text? Alt text allows screen readers to read information about images out loud to users who are blind or and visually impaired. Alternative text is necessary so that people who are blind or visually impaired can still access the information provided by an image and understand the purpose of the image. In the below image the ALT tag is described simply as ALT tag. There is also a written version below.