var lf = document.getElementById('login-form');

var p = lf.pwd;
var u = lf.login;
        
var b = function() { 
  if (p.value == '') { 
    p.style.background = "#ffffff url('./style/login_text_heslo.png') left 5px no-repeat"; 
  } 
  if (u.value == '') { 
    u.style.background = "#ffffff url('./style/login_text_prihl.png') left 5px no-repeat"; 
  } 
}; 
 
var f = function() { 
  p.style.background = '#ffffff'; 
};
var f2 = function() { 
  u.style.background = '#ffffff'; 
};

function login_background() {
  p.onfocus = f;
  u.onfocus = f2;
   
  p.onblur = b;
  u.onblur = b;
  
  b();
}

login_background();