If you use an image to link to another website, you should use an alt attribute in the img tag. Google will use the alt attribute as a replacement for the regular anchor text. That’s what Google’s John Mueller said in a webmaster hangout:
“I think is is useful there is either that you have the text link as well or at least that you use the alt attribute for the image to give us the anchor text that you want associated with that link.”
You can view the video here:
Check the image alt attributes on your web pages
Among many other things, the website audit tool in SEOprofiler checks the img alt attributes of the images on your website. Check your web pages now: