CSS to center images unless I say otherwise? My images keep moving left

I have gone through and edited the same posts over and over. I center my image and the next time I see this post in my blog my images are all over to the left again. Could this be the theme? I’m using Atahualpa 3.4.6.

Is there CSS I can add to always center the images unless I say otherwise? This is not the worst thing that ever happened in the history of the world, but it is annoying me.

There is a place in the theme to put CSS, but I’m not a CSS expert.

Thank you for any suggestions.

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