var schoolURL = "imgs/school1.jpg";

if (document.images) {
     var school1 = new Image();
     school1.src = "imgs/school1.jpg";

     var school2 = new Image();
     school2.src = "imgs/school2.jpg";
	 
	 var catalog = new Image();
	 catalog.src = "imgs/catalog.jpg";
	 
}

function changeSchool() {
     if (document.images) {
        if (schoolURL == "imgs/school1.jpg") schoolURL = "imgs/school2.jpg";
		else if (schoolURL == "imgs/school2.jpg") schoolURL = "imgs/catalog.jpg";
		else if (schoolURL == "imgs/catalog.jpg") schoolURL = "imgs/school1.jpg";
		
         document.mySchool.src = schoolURL;
	}
}

function changeSchoolBack() {
	if (document.images) {
		if (schoolURL == "imgs/school1.jpg") schoolURL = "imgs/catalog.jpg";
		else if (schoolURL == "imgs/school2.jpg") schoolURL = "imgs/school1.jpg";
		else if (schoolURL == "imgs/catalog.jpg") schoolURL = "imgs/school2.jpg";
		
		document.mySchool.src = schoolURL;
	}
}