Often when posting a link on Facebook, a picture and description will show up in the FB post. However somtimes the image will not be the what you expected or wanted. This is especially a problem i you use som kind of auto-posting feature on website, such as JetPack for wordpress or similar services, because in these cases you do not see the post… unless you actively visit you Facebook page and check.
Facebook or other social medias, like Twitter and Google+, rely on meta data to show the image. Google prefers schema.org, but Facebook rely on Opengraph tags. In CMS’s like WordPress these tags are often added automatically by the theme or various plugins. Sometimes the problem is caused by multiple tags, and in this case you have to figure out which plugins and themes does what. You can start by looking at the source code and seach for “og:” which will show you the Open Graph tags. You have to make sure the correct image is tagged before you can solve problems with links already posted to your FB page.
Once you know your website is properly tagged, there is a way to force Facebook to use the correct image. All you have to do is follow these steps:
Paste the link to page of your website into the Facebook Open Graph Object Debugger and click the “Fetch new scrap information button”: https://developers.facebook.com/tools/debug/og/object/
This forces Facebook to clear the cache for your webpage, however for the actual posts on you FB page to reflect the change, you have to “refresh the share attachment” on the post that shows the wrong image.
First click the date on your post
This takes you to a page with only this one post. Next click the little arrow in the top right hand corner and choose “refresh the share attachment”
Note that the “refresh the share attachment” only shows up in the post page view, i.e. you cannot do this from the timeline/pagefeed view
If this does not set the right image, you have either forgotten to Fetch new scrape information in the Debug tool above, or you page is not tagged with the correct Opengraph tags