An img element must follow the guidelines of Success Criterion 1.1.1 (Non-text Content), by following either of the two situations:
This img element does not contain an alt attribute, and as such assistive technology cannot make any assumptions as to whether it is a "decorative" image or not. The handling of such an image is therefore undefined.
This is documented as explicitly failing the Success Criterion under Common Failure F65, "Failure of Success Criterion 1.1.1 due to omitting the alt attribute on img elements, area elements, and input elements of type 'image'".