Centering Custom Header

I am brand new to WordPress and really only know a TEENY bit about css… So, I have no idea how to fix this. The original header for the theme stretched all the way across the page, yet the dimensions called for for my custom image appear to be way short. I even tried resizing it wider, but was still forced to crop it down. Since that is the case, I really just want the header centered.

Here is the snippet of code that I think applies:

#masthead {
background: #180509;
}
#header_text {
position: absolute;
top: 35px;
left: 40px;
width: 950px;
}
#masthead a {
color: #fffff4;
font-size: 1.7em;
text-decoration: none;
}
#masthead h2 {
color: #fffff4;
font-size: 1.2em;
}
#headerimg {
margin-left: auto;
margin-right: auto;
}

I do not even see my header image in the above coding, and it was nowhere else to be found in the rest of the code. Anyway… Can someone tell me how to center my image?

Thanks so much!