Centering header img

Hello,

I can’t seem to figure out how to center the header img without using padding. I obviously don’t want to use it because its inconsistent between devices. When i add a width property it just makes the img larger… Here’s my css code thus far:

.site-header img {
display: block;
margin-left: auto;
margin-right: auto;
position: relative;
}

.site-header {
display: block;
text-align: center;
position: relative;
}

help is greatly appreciated