Images can be cached in your browser and if you site is running a cache, on the site cache also. When an image is deleted and replaced with an image with the same name in the same folder, usually the browser will display the old version. But the new version is actually on the server and will eventually show up. The way to make the new version of the image to show up immediately is to give it a different name. Cleaning your caches after uploading a file usually resolves cached image issues.
As for deleted images, I’m not aware of any process or steps one could follow to make image disappear from the server. If you discover what things were done to cause this to happen, please let me know.
