Image not displaying

I’ve got a single image in the post on my site: (https://tartary.f2o.org/wordpress ) that refuses to display. (it’s the one in #8). It shows the alternate text, and that’s it. I’ve tried reuploading, changing the file name, etc, to no avail. Anybody have any ideas?
Also: what’s the best way to center-align images, without wrapping text around them?