.attendsubmit
 {
  width:  25px;
  height: 25px;
  margin: 0px;
  padding:0px;
  cursor: pointer;
  display: inline-block;
  filter: invert(50%);
  -webkit-filter: invert(50%);
  background: transparent url('/img/ok.png') no-repeat center top;
}


input[type=checkbox]:checked + .attendsubmit  {
  filter: invert(0%);
  -webkit-filter: invert(0%);
}