The image is presently 922 pix so get the designer to make it 1080 and see how you go, these things can be a bit of trial and error and on some sites I have included a background to take up the space by adding it into the header like this
Code:
<td align="$stylevar[left]" background="$stylevar[imgdir_misc]/headerbg.gif"> </td>
<td background="$stylevar[imgdir_misc]/headerbg.gif" align="$stylevar[center]"><a href="$vboptions[forumhome].php$session[sessionurl_q]">
<img src="$stylevar[titleimage]" border="0" alt="$vboptions[bbtitle]" /></a></td>
<td align="$stylevar[right]" background="$stylevar[imgdir_misc]/headerbg.gif"> </td>
Try the attached image I have resized it as suggested and see how you go.