SRC and ALT are required for IMG in html. There are a lot of alerts in my SEO Tool too.
Quick fix for my records and twimc:
plugins / content / jdownloads / jdownloads.php
Line: 846,915,927,1634,1665,1674 search for
and set to
an other possibility would be to set the alt tag to alt="" (NULL) for accessibility. (https://www.w3.org/WAI/tutorials/images/decorative/
) The whitespace is the problem.