Feel free to change medium to whatever size you want Facebook to fetch. In fact, we can add to the config.xml file: <images> <image reference="facebook" maximum="150" quality="80" default="facebook_default.png" />Make the edit in main.php {$PRODUCT.facebook}. Create a new default image that is useful for Facebook viewers that suggests there is no image for this product, such as, instead of "No Image Available", have the image be of an "Under Construction" sign or "Check for this im