Hi,
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
alt=" "
and set to
alt="'.$files[$i]->file_pic.'"
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.