I would suggest either you should use white background in your dashboard to tackle this situation or crop the white space from your logos using some photo editing tools like photoshop. If you see around your logo images there is white space which can not be transparent with existing sheets background. Try to download the exact logo whitout white space or change the background to none.
Thanks for the quick reply Mahfooj Khan! Well, the logo images I loaded up where made with a transparent background and not white. Yet when Tableau displays it, they show up as white.
Anything worked Orlando ? Facing the same issue.