Hi Kristen,
yes we've had the same thing happen. It's been escalated already. I think it has to do with the new "upgrade" Marketo rolled out last weekend and they are currently working on the fix but won't likely have one until next week. They have suggested instead of swapping the image, I delete it, then add a new one. I have not tried it yet though. I will try that later today, and keep my fingers crossed...