When an image is decorative

An image is decorative when it carries no information the reader would otherwise miss: a chapter ornament, a rule, a repeated flourish, a spacer. The correct treatment is an empty alt attribute (alt=""), which tells assistive technology to pass over it in silence.

The difference that matters

A missing alt attribute and an empty one are not the same thing. A missing attribute makes a screen reader announce the file name, which is noise. An empty attribute is a deliberate instruction to skip. Never delete the attribute to mean "decorative".

<img src="ornament.png" alt="" role="presentation">

When it is not decorative

If the ornament differs per chapter and the difference means something, or the flourish is the only thing marking a scene break, it is content. Describe it once, briefly.