Issue with image center alignment

Try this CSS:

.post-content .aligncenter, .post-content .aligncenter img {
   display:block;
}

To add or override CSS: use the “Additional CSS” option in the customizer. https://codex.www.ads-software.com/CSS#Custom_CSS_in_WordPress

Learn to use the Chrome Developer Tools or Firefox Developer Tools to help you see and test changes to your CSS.

Thank you very much! That worked perfectly. Is there anywhere I can read up about why it did not work and why does it work now? I dabble in the code from time to time but would like to know how to do the basics.

Anyway: thanks!

That CSS has nothing to do with WP core; I suspect some issue with your theme or an interaction with your theme and plugins. I recommend asking at https://www.ads-software.com/support/theme/lovecraft#new-post so the theme’s developers and support community can help you with this.