An image with the alt attribute left off

The image below carries no alt text. A screen reader moves past it as though it were invisible.

What alt text does

Without alt, a screen reader meets a hole in the page and a crawler meets silence. The software speaks the alt text in place of the picture, so the picture needs words. If the image means something, one short, specific line about its subject does the job. If it is only decoration, an empty alt marks it as something the software should pass over by choice.

Writing the alt text well

Name the subject and the job it does on the page; the words "picture of" belong in neither. Keep the line short and free of keywords, and keep the attribute itself. An empty alt and a missing alt look nearly alike in the source, yet whatever parses that source reads them as opposite instructions.