Has your instance been updated to ssl/https yet? Meaning - are your landing pages and images on https as default?
I had this issue with one of my clients too but after we updated to https it was all good. However, it also seemed that we only had issue on one particular domain, so DNS can also be part of the problem here.
Hi, Peter thanks for the reply. As a matter of fact, the images URL have HTTP, this might be the issue. Do you know if I have to contact Marketo to make the switch to HTTPS, or I can do it through the Admin?