All images must have an alt attribute! This should not necessarily be a description of the image, but rather a substitute for it.

For example: If the image is a face that is either happy or sad based on input, descriptions like "Yay! Great input!" and "Ew, that was bad input." might potentially be better descriptions than "Smiley face" and "Sad face."

If the image is only there for stylistic reasons, such as a doodle separating two paragraphs or an arrow indicating the direction to read, you can use an empty alt attribute to indicate that this image does not provide any additional information to the reader. There is no reason to force the user to listen to a lot of text that does not help them move forward.

Inline SVGs does not have alt attribute, so for them you can add an aria-label instead:

<svg aria-label="Description of the SVG">
  ...
</svg>