Float-header. {background-image - how to center?

I set the logo and menu centered and I would like to add a partially transparent picture under float-header.

(I would like that the sides of the image are transparent and smoothly pass into a full filling in the middle where they are under the logo - a normal gradient on png or gif.)

The problem with this is that at lower resolutions this image always runs to the right - it is not centered - I am missing some entry in css. Help me please

.site-header.float-header {
		background-color: transparent;
		background-image: url("../domain.com/header_float_bckg.png");

}

Hello, I’m not sure what is your issue.

Can you please provide some explanatory screenshots and a link to your website?

Kind Regards, Roman.