There was a very small issue with the DS-Syndicate module that broke the W3C Validation compliance. The img tag did not end with a forward slash. To fix this, simply open /modules/mod_ds-syndicate.php and on line 51, column 64 add a / to the end of the <img> tag so the line looks like:

$mod_content .= "<img src="" . $feed_img . "" alt="feed image"/></a>";