My no-CSS-upgrade solution is an invisible linking rectangle in the dimensions of the header image, placed exactly in front of it. For the standard version of the theme, that would mean putting this in a text widget:
<div align="center" style="width:100%;top:73px;left:0;position:absolute;z-index:11;">
<a href="YOUR BLOG URL HERE">
<div style="width:760px;height:190px;">
</div></a>
</div>
But you've changed the dimensions, so you'll have to change the numbers. The width and height of the inside div should be those of your header image; if you can't calculate the proper top and left numbers for the outside div, try this first (a red rectangle):
<div align="center" style="width:100%;top:73px;left:0;position:absolute;z-index:11;">
<a href="YOUR BLOG URL HERE">
<div style="background-color:#f00;width:760px;height:190px;">
</div></a>
</div>
Adjust the numbers till it falls in place, then delete the bg color.
Thesacredpath or hallluke will probably be able to give you the CSS-upgrade variant of this solution or suggest an alternative one.