However I'm guessing that when you first set it up, you were using the local IP to connect to it. And I'm also guessing that you uploaded the pictures to the server using the LAN IP (rather than the URL that is now serving it). I'm not very familiar with WordPress but it sounds to me like WordPress is storing the full URL for the uploaded content (which contains your LAN IP - not the DynDNS URL).

I did a quick google and discovered a WordPress doc page that looks like it will help you fix your issues: https://codex.wordpress.org/Changing_The_Site_URL

If you change your WP site URL to the DynDNS URL then it should all work properly.

For future dev work on your server and assuming that you are using a desktop (or a laptop that is ALWAYS on the LAN) then I would recommend that you add a "hosts" entry for the DynDNS domain that points to the local IP (google "how to add a hosts entry" for your OS). That way when you upload/download stuff from your site it doesn't have to go out onto the internet and loop back around back into your LAN (will just go direct to/from the local IP). However I don't advise that you do that on a laptop; as you'll possibly forget and wonder why you site isn't working when you are out and about! (It won't be able to connect to the local IP).