function changeDivImagea() 

{ 

    document.getElementById("display").style.background = "url(./portfolio/tra4a.jpg)"; 

}
function changeDivImageb() 

{ 

    document.getElementById("display").style.background = "url(./portfolio/tra4b.jpg)"; 

} 
function changeDivImagec() 

{ 

    document.getElementById("display").style.background = "url(./portfolio/tra4c.jpg)"; 

} function changeDivImaged() 

{ 

    document.getElementById("display").style.background = "url(./portfolio/tra4d.jpg)"; 

} // JavaScript Document