The first time Facebook accesses your webpage, it will cache the image and text it finds. Facebook then prefers to use the cached information until it has expired (which could be never). So, before you hit the Facebook Send / Share button for the first time, make sure you’re satisfied with your Post or Page images and text. If you change your mind, and your webpage has not been liked or shared yet, you can use Facebook’s Open Graph debugging tool to refresh Facebook’s cache. If your webpage has already been liked or shared on Facebook, then there’s nothing you can do to change the title, descriptive text, or image that was used.
After publishing a new Post or Page, a ‘Validation Tools’ tab will appear in the Open Graph+ Custom Settings metabox. You can use these links to check the content that is extracted from your webpage by Facebook, Google, etc.
See Facebook’s Maximizing Distribution for Media Content article for more information on open graph meta tags and image sizes. Facebook recommends an image size of 1200x630px, 600x315px as a minimum, and will ignore any images less than 200x200px.
NGFB Open Graph+ will also ignore images that are smaller than the Image Dimensions defined on the Open Graph+ General settings page. Always make sure the original, full size images uploaded to the WordPress Medial Library are larger than the Open Graph+ Image Dimensions you have configured. If the plugin cannot retrieve an image of the correct size from the Media Library, the image will be ignored / rejected by the plugin.