Do these images get updated often with new titles or synopsis? 

Old revisions of assets get purged after a time, and when Images are purged, the storage goes away with them.

There is a configuration value in your WebGUI.conf file called "richEditorsUseAssetUrls" that will fix this problem when adding new images using the rich editor. However, it comes at a significant performance penalty.

Otherwise this is all functioning as intended.

Closing as not a bug.