This is what happens when Google suspects the image you are showing to visitors is different to the one you are showing to the Google crawler.
Obviously some webmasters may do this in order to, for example, show adult content without the search engines realising what it is (as they can recognise adult content images).
But most of the time the mistake is innocent. It can be caused by:
If you have a system to stop hotlinking you may need to look around for another similar service that is more search engine friendly.
For systems that try to stop visitors stealing images they are ultimately pointless. Shift + print screen allows anyone to take a picture of the entire screen and then clip it in almost any image editor. A better option would be to water mark your images with your brand or domain name.
I'm Tim Hill, a Search Engine Optimisation and Online Marketing specialist. I created this site to help others understand that SEO is not a mysterious black art!.
If your a newbie try the Getting Started in SEO page, otherwise feel free to dig around and learn more.