Some web authoring software automatically inserts the file name and file size into the ALT TEXT properties: pretty pointless when you think about it. The photo below has these features.
Without helpful alt text, screen readers are unable to convey what the graphic is. With graphics that are essentially page decoration, this might permissible. However, the graph below, which expresses information in a convenient way for sighted people to understand, has no alt text. The data on the graphic is invisible to screen readers.
The graphic above is invisible to screen readers: it has no alternative text to explain its content to visually impaired readers.