Не работает функция backgroundImage, c backgroundColor проблем нет

function change(ob)
            {
                var tbs = document.getElementsByTagName("td");
                for (var i=0; i<tbs.length; i++)
                {
                    if(i == ob.cells.value-1) 
                    {
                        change_color(ob, tbs, i) 
                        change_background(ob, tbs, i) 
                    }
                }
            }
            function change_color(ob, tbs, i)
            {
                switch(ob.color.value)
                {
                    case 'w': {tbs[i].style.backgroundColor = "#FFFFFF"; break;}
                    case 'r': {tbs[i].style.backgroundColor = "#FF0000"; break;}
                    case 'y': {tbs[i].style.backgroundColor = "#FFFF00"; break;}
                    case 'g': {tbs[i].style.backgroundColor = "#008000"; break;}
                    case 'b': {tbs[i].style.backgroundColor = "#0000FF"; break;}
                }
            }
            function change_background(ob, tbs, i)
            {
                switch(ob.pic.value)
                {
                    case '0': {tbs[i].style.backgroundImage = ""; break;}
                    case 'j': {tbs[i].style.backgroundImage = "java.png"; break;}
                    case 'js': {tbs[i].style.backgroundImage = "js.png"; break;}
                    case 'c': {tbs[i].style.backgroundImage = "c.png"; break;}
                    case 'p': {tbs[i].style.backgroundImage = "python.png"; break;}
                }
            }`введите сюда код`