Where to add Header Banner code?

Hi there,
My website: https://www.radicards.com/
I used to have my custom banner coded into the style.css file of this theme. The banner was located between the rows for categories and pages. I recently had to reinstall the theme and I lost my custom banner. I tried re-including the code into the style.css file just under for following line:

#header #headerbanner a:hover {
background: url(images/blacktrans.png);
}

I added the following code just under what’s shown above and this code used to work. Does anyone here know what I need to do to reinstate my custom banner between the rows for categories and pages at the header of this theme?

#header {
background-image:url(‘IMAGEURL’);
background-repeat:no-repeat;
padding-bottom:40px;
height:105px;
}
#header #logo {
display:none;
}
#headerbanner {
display:none;
}