Javascript checkbox to change background not working

I'm trying to change the background position of an element using a checkbox, but is not working.

You can see the file here: http://ift.tt/2hWwSuq

Here is the code:

<div id="checkboz" class="checkbox">
            <label for="modlgn-remember"><input id="modlgn-remember" name="remember" class="inputbox" value="yes" type="checkbox">Recordarme</label>
</div>

JS:

  x=false;
              function Check(){
                  if(x){    
                  document.getElementById("checkboz").style.backgroundPosition='0 0';
                  document.getElementById("checkboz").style.backgroundPosition='-28px 0';
                  x=false;
                  }
                  else{
                  document.getElementById("checkboz").style.backgroundPosition='none'; 
                  document.getElementById("checkboz").style.backgroundPosition='0 0';   
                  x=true;    
                  }

          }

CSS:

#checkboz{background: url(http://ift.tt/2i2lMRP);
background-size: 55px;
background-repeat: no-repeat;
width: 28px;
height: 28px;}

Thank you so much for your help :)