how to remove duplicate images in email

I've created an email with an image thumbnail that will link to a video.  When I send out the sample, I can see the email just fine on my Mac.  But, some others show a duplicate image of the thumbnail:

I've done some searching on the internet and in the Marketo's support pages, but couldn't find anything other than issues with "hero" zones and images (which I'm new to Marketo and even the "hero" thing had no real information on Google or in the support pages)...so....any ideas that would help with this issue would be greatly appreciated.

Thank you.