When testing your page for Open Graph tags, the best tool we'd recommend is Facebook's own debug tool:
developers.facebook.com/tools/debug
That will show you all the tags on your page. The way open graph works is that the first og:image tag found *should* be the one Facebook uses. However, they are also picky about the size and content of an image, so they may choose the second image instead (which is better than them picking a random image from your page if they won't use the first). The Debug Tool will generally tell you why an image may not be used (too small, too much text, etc).
On a specific page, if you want to set your own image, you can use the following tag in the article:
{SCOpenGraph image=https://domain.com/link-to-image.jpg}
I would not do anything from the forum post you mentioned. It's 4 years old, which means it likely won't work, may break something and likely is just wrong now.
I hope the above helps, but if you need anything else, please let us know.
Thanks,
Alex
Multiple og:image tags in article. Facebook doesn't show correct image
