Facebook loves changing their platform and API’s and discovering the correct way to do things isn’t always easy.
So there used to be a sharer.php link you would pass parameters to, including images and other things that would be in the post.
This has been replaced with the below version which seems to have much more limited capabilities.
You can read more about it on the Facebook developer center.
Note there is also an Open Graph method which would likely use any open graph tags within a page to use in the post, such as images, titles and descriptions.