Can't get images to align to the centre since upgrade

Since a recent upgrade I have started to create a new page, but cannot get the images to align to the centre, either through the visual or html versions of page editing.

The website https://www.lilacconference.com has existing pages with centre aligned images, but the page I’m working on at the moment won’t do it, and I can’t see what I’m doing wrong.

I don’t know enough about CSS to know if it’s something in the theme which is the issue, but I’ve been able to do it in the past!

Thanks,