Hello kstyles,
Regarding the masthead image: It is not possible (nor desirable) to stretch an image depending upon the width of the screen. Instead, set the image to be aligned centrally within the screen and set the background to be the base or bleed of the masthead.
<div id="masthead">
<img src="masthead.jpg" />
</div>
#masthead{
margin:0px auto;
text-align:center;
background:#00a;
height:50px;
}
You may need to slightly redesign the masthead image so that it has the same colour at the left and right hand edge and therefore fits with the background colour that you have set.
I hope that helps as a starting point.
Regards,
David