A gallery with 1 image (thumb size) & centered?

Hello. I had been able to create a wordpress gallery with 1 image only, thumbnail size, attached to the media file, and centered the gallery in a DIV to have my one thumb centered on the page. It worked beautifully.

An hour later, without touching it meantime, it stopped working. And will no longer work no matter how many times I re-do the gallery and the centering.

Instead of thumb size, the image now insists on appearing full-size, and defying the DIV centering, it’s shoved to the left.

It just occurs to me, I’m also using the Tiled Galleries Carousel Without Jetpack plugin to give me square thumbs and circular thumbs for wordpress galleries (without Jetpack).

Any ideas? Thanks.