@MisterE Alt (alternative) text is used to describe the image. it's used by screenreaders.
You should use it for any image really, when posting.
On a webpage its good to fill in the alt text if the image is conveying information, relevant to the content on the page. If its just for decoration, filling in the alt text is not required.
https://webaim.org/techniques/alttext/
@MisterE With difficulty. Especially if an image is a link. Without alt text, a user would have no idea where they were going.