CP 6.1 publish error - graphics are missing and appear as a red box after publishing

I am using CP 6.1 on Mac OS 10.8.3. When I publish my course and review the published file,  some BUT NOT ALL of my images appear to be missing and are replaced by a red box. This does not happen EVERY time I publish but frequent enough for it to be an issue. When I open the same CP file from a co-worker's computer (also using CP 6.1) and publish, the error does NOT occur and all images appear as expected. What is going on with my version of CP? Has anyone else experienced a similar issue?