Script Header Image

You have to add the custom JavaScript to the theme (probably best in the footer) then use or create an ID for the header that will be used to change the image.

The custom javascript is as follows:

<!--

function newImage(arg) {
	if (document.images) {
		rslt = new Image();
		rslt.src = arg;
		return rslt;
	}
}

function changeImages() {
	if (document.images && (preloadFlag == true)) {
		for (var i=0; i<changeImages.arguments.length; i+=2) {
			document[changeImages.arguments[i]].src = changeImages.arguments[i+1];
		}
	}
}

var preloadFlag = false;
function preloadImages() {
	if (document.images) {
		Videos_over = newImage("https://sites.google.com/site/thestampinschach/Videos-over.png");
		Videos_Rewards_over = newImage("https://sites.google.com/site/thestampinschach/Videos-Rewards_over.png");
		Rewards_over = newImage("https://sites.google.com/site/thestampinschach/Rewards-over.png");
		Rewards_Join_over = newImage("https://sites.google.com/site/thestampinschach/Rewards-Join_over.png");
		Join_over = newImage("https://sites.google.com/site/thestampinschach/Join-over.png");
		Join_Specials_over = newImage("https://sites.google.com/site/thestampinschach/Join-Specials_over.png");
		Specials_over = newImage("https://sites.google.com/site/thestampinschach/Specials-over.png");
		Specials_Products_over = newImage("https://sites.google.com/site/thestampinschach/Specials-Products_over.png");
		Products_Specials_over = newImage("https://sites.google.com/site/thestampinschach/Products-Specials_over.png");
		Products_over = newImage("https://sites.google.com/site/thestampinschach/Products-over.png");
		Awards_over = newImage("https://sites.google.com/site/thestampinschach/Awards-over.png");
		preloadFlag = true;
	}
}

// -->

Just add it and modify accordingly.