This screenshot shows the sidebar in explanation for "Linked image missing alternative text". It reads:
Reference
Errors
Linked image missing alternative text
What It Means
An image without alternative text results in an empty link.
Why It Matters
Images that are the only thing within a link must have descriptive alternative text. If an image is within a link that contains no text and that image does not provide alternative text, a screen reader has no content to present to the user regarding the function of the link.
How to Fix It
Add appropriate alternative text that presents the content of the image and/or the function of the link.
The Algorithm... in English
An image without alternative text (missing alt attribute or an alt value that is null/empty or only space characters) is within a link that does not contain text and an image with alternative text.
Standards and Guidelines
Icon index