Hello,
I think I can guide you in the right direction.
I took a look at both the map view and the list view on your site and the coding they both use the same function so I would expect if that was broken both would be.
I did noticed on the List view the image url is different and looks like this:
http://i2.wp.com/5kevents.org/5k/wp-content/uploads/2014/08/iron_horse_newweb.jpg?fit=151%2C150
That i2.wp.com is from Jetpacks CDN, so if you can turn that off that might help.
Another area to check is any htaccess security coding or a security plugin, or your hosting service that is preventing the site from showing the images from the CDN.
I hope that helps get you in the right direction, let us know if have trouble.
Thanks