how to insert header image w/o the option?

i’m using the Untheme two-column on https://www.annapolitanbride.com
and there is no option to insert a header image.

is there a way to do this in the style sheet instead?

this is the header code from the style sheet:
/***********************
* Header *
***********************/
#headerimg .description {
font-size: 1.2em;
text-align: left;
}

#header {
background-color: #666;
margin: 0;
padding: 10px;
height: 80px;
width: 940px;

}