It's happening the same problem with me and I didn't change anything about hostname.
I've had supercache and cnd_sync_tool. I've decided to make a test with W3TC, because I'm very interested using memcache mode, but it isn't working on CDN. So, I've uninstalled supercache and cdn_sync_tool and I installed W3TC.
All my images (from media library) are displayed only my cdn hostname, like that:
<img width="300" height="120" src="http://cdn.xxxxxxxxx.com/" class="attachment-highlight wp-post-image" alt="" />
The system has changed all image paths to only cdn hostname.
I made some tests and the wordpress returns the right image infos. I could receive the right ID using get_post_thumbnail_id(). The problem is in replace hostname function.
I've made another test too. I've uploaded a new image and put it in a post, but the problem continues, but I may see it on media gallery admin without problem.