Solved!
The issue was in light_responsive/mail/html/mail_header.tpl
Commented out:
{include file="mail/html/responsive_row.tpl" content="<img src='`$AltImagesDir`/custom/logo.png' alt='' />"}
As so:
{*include file="mail/html/responsive_row.tpl" content="<img src='`$AltImagesDir`/custom/logo.png' alt='' />"*}