Featured Post Images are used when syndicating your posts to LinkedIn, Twitter, & your own WordPress pages.

The 1st Post Image Within Content is used when syndicating your posts to Instagram, Facebook, Mastodon, & Pixelfed.

Why Are Different Images Used For Different Social Media Accounts?
This is an outcome of the plugins used for syndication. Mastodon and Pixelfed image syndication settings offer greater flexibility and can be customized using the ‘Share on Mastodon’ and ‘Share on Pixelfed’ plugin settings pages.

Instagram, Facebook, LinkedIn, & Twitter image syndication behaviors are fixed and non-configurable.