If you add Facebook like/share buttons to your page, people can share your page to their Facebook wall. Facebook will pick the image and description based on the open graph meta tags specified on the page. In some cases, you may find that even if you have specified the open graph description and image tags, the image shared on Facebook is different.
To debug and fix this issue, Facebook provide a debugger. Visit Facebook debug tool and debug the URL to the page where you find issues, and Facebook will show you the possible cause.