The issue with some links is that the Facebook scraper, which we use to build link previews, returns invalid images to us. Facebook's debugger shows data, but the API scraper does not. Unfortunately, we can't build a proper link preview when the API scraper doesn't return data back to us, since we can't predict what Facebook will show once the post is published.
Even so, if a preview doesn't show up on GAIN it will most certainly show up on Facebook once the post is published. We understand this situation isn't ideal so there are a few workarounds:
Make sure Open Graph meta tags are correct.
Facebook's recommendations for using Open Graph meta tags on your website can help maintain control over how your links will appear once published. With explicit OG tags in place, we can for sure create a proper preview. For more information on using Open Graph meta tags on your website, visit this page.
Use photo post format
Upload your preferred image to your post and paste any links in the post message field.
Share directly on Facebook's UI
If you need to have an exact idea, for a specific client, of how Facebook will display a link when published, try to share it directly on the Facebook UI and see what comes up.