Center Header Image on Page

There seem to be 100s of solutions to centering the header image on my WP site, using Twenty Twelve theme. I have tried many and read many posts. I’m thinking there must be something else on the site blocking these centering instructions.

In Additional CSS tried:

#header {
text-align: center;
}

and
.header-img img{margin: auto;}
and

#header-logo-image {
    text-align: center;
}

none work…

Thank you for your attention.

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