Center Feature Image

Hi there. My site is https://www.simplysabrinablog.com and I am looking to center my feature image.

I was running into a problem last night where all of my *new post* images which were uploaded at 504 x 640 were bring resized to the container size at 800 x 760. I couldnt find the stylesheet to overwrite this, so I added custom CSS in the form of:
——————————————————
“img {
max-width: 504px;
}”
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which made all of my images have a max width of 504, which wasn’t a huge deal since that is the standard for my posts. However, it did make my home page feature image smaller. If I have to keep the feature image smaller and not fill the size of the container (800px) then I at least want to center it.

Can anyone help please?