Phoca Open Graph uses the following process to display images in open graph meta tags (og:image):
1) Image which is set in parameters of the plugin (plugin parameters) will be set for the image meta tag on the website. In case, parameter Image (Parameter) Order is set to value First.
2) If no image is set in plugin parameter then the image set in category parameters (category of the article).
3) If no image is set in category parameters then the image set in article parameters - in parameter image_intro is set.
4) If the image is not set in parameter image_intro then the image set in article parameter image_fulltext is used.
(When you edit an article in Joomla!, you can set the images for Intro or Fulltext).
5) If no image is set in plugin parameters or article parameters, Phoca Open Graph plugin tries to find an image inside the article. If some is found, such will be used in og:image meta tag.
6) If the image is not found in article, then the folder on the server: images/phocaopengraph will be searched. So if there is image inside this folder, this will be used. Use article ID to name the image. So if the article ID is e.g. 36, the following image will be displayed: images/phocaopengraph/36.png.
7) Image which is set in parameters of the plugin (plugin parameters) will be set for the image meta tag on the website. In case, parameter Image (Parameter) Order is set to value Last.
Don't forget that this plugin is a content plugin and it can render the meta tags only on article sites (com_content). In article view or in blog view where more then one article are rendered).
This flow was voted by users and it can be easily changed in the code (plugins/content/phocaopengraph/phocaopengraph.php).
The flow in short:
Image in Plugin parameters -> Image in Category parameters -> Image in Article parameter (Intro Image) -> Image in Article parameter (Fulltext image) -> Image inside article content -> Image inside images/phocaopengraph folder on the server.