Facebook detects the images using an algorithm that we cannot control unless specified by Open Graph tags in the page's Page Tags field. If the image is larger or smaller than specifications, Facebook will often display the default image supplied by the module instead of the image that is in the item itself. Please look at the dimensions of your photo in the item and ensure that the image has proper specifications to post to Facebook.
Use images that are at least 1200 x 630 pixels for the best display on high-resolution devices on Facebook. However, coming through RSS feed, Facebook recommends reducing the image to the minimum size.
Minimum Image Size: The minimum image size is 200 x 200 pixels. If you try to use an image smaller than this you will see an error in the Sharing Debugger.
Maximum Media File Size: 3 MB
Supported Media Types: jpg, png, gif
Also, see the Facebook Sharing Best Practices Guide to troubleshoot specific post specifications.