I'd call support or post screenshots.
Is the image still there inside Design Studio > Images?
I had this same issue a few weeks ago, try calling Marketo Support. They might say its your IT department, so check with them as well.
Send yourself a sample email. Look at it in different email clients (Outlook, gmail, yahoo, etc.). Send it to a co-worker. if the image is not appearing in all those situations, then something's probably wrong with your image. Is it displaying for your co-worker or in your personal email? In that case, it could be something with your settings, and you should check with your IT team.