Hi friend,
You've got the problem named "mixed-content" in your wordpress site. It indicates that your resource links have some "non-https" link and the browser of client will warn the insecure completely for your site.
So you have to change all your resource links to "https", maybe in your code or in your database, or you can try to use a plugin to convert all link to https. You can use plugin named "SSL Insecure Content Fixer" in Wordpress to fix it.
So you said "third party images", i think they are external links. The most important you need to care that the external host have to support "https" too. If they are not, you will fail in setup or fix this problem guy.
Regards.