If you have a secure page (HTML delivered via HTTPS) that calls an image or any resource from a source that is not secure, the page will now be considered overall not secure. ... Mixed content affects the way your webpage is displayed by browsers.
WordPress stores the full image URL in its post database.
So after turning the website to HTTPS, you have to rewrite every image src in your posts with the new HTTPS address. In backoffice, just deleting old image and replace it with the same should do the trick.
If you have a large number of posts, some plugins could help you convert HTTP to HTTPS. Or ask a developer to replace them in the database.
Then, depending on the Theme you are using, the developer should also replace links to scripts, CSS... with HTTPS. But if you are running an official or recent Theme, it should already be good.