drugnormal_image = new Image();
drugnormal_image.src = "http://www.sparrowvswallow.com/help/workspace/sysimg/drug_1.jpg";

drugmouseover_image = new Image();
drugmouseover_image.src = "http://www.sparrowvswallow.com/help/workspace/sysimg/drug_2.jpg";

buynormal_image = new Image();
buynormal_image.src = "http://www.sparrowvswallow.com/help/workspace/sysimg/buy_1.jpg";

buymouseover_image = new Image();
buymouseover_image.src = "http://www.sparrowvswallow.com/help/workspace/sysimg/buy_2.jpg";


<!-- repeat the 4 lines above for any subsequent images. -->

function swap(){
if (document.images){
for (var x=0;
x<swap.arguments.length;
x+=2) {
document[swap.arguments[x]].src = eval(swap.arguments[x+1] + ".src");
}
}
}