Each image should have an alt attribute. If the image is meaningful- the alt text must convey the same meaning as the image, e.g. when an image contains words that are important to understand the content, the alt text should include those words. In case the image is decorative and should be ignored by assistive technology the alt should be set to null.
Engine scans the page for
<IMG> elements inside links missing an alt attribute
Users who use assistive tools, such as screen readers rely on the alternative text.
<img src="/example_img.gif" alt="example_text to link" />