I definitely set the alignment of the Container Block holding it to center.
I have the image (my signature) at the end of the post instead of in the query loop because there will be another author besides me, so the image signature will need to change from post to post. (David said in a different post that setting a dynamic custom signature for each of the two authors wasn’t possible so I resigned myself to add it manually for each post).
I removed caching plugins, and see that the image is centered properly on Single post pages, but not on the front end of pages containing query loops. (The image is centered in query loops in edit mode). I hope this information helps diagnose the issue.