css form checkbox styling with checked and after tags

I am trying to design a form without using JavaScript or JQuery. It includes a series of checkboxes. The idea is to display a certain gif after the checkbox if it is unchecked. Otherwise, display nothing after it. This is the code I have:

input[type=checkbox]::after
{
  content: url(images/unchecked_after.gif);
  position:relative;
  left:15px;
  z-index:100;
}
input[type=checkbox]:checked::after
{
  content:"";
}

It works in Chrome but doesn't seem to be working in Firefox or IE. What is wrong?