<script type="text/javascript">
function machGroesser(e) {
   e.removeAttribute("width"); 
   e.removeAttribute("height"); 
}                

<img src="./test.gif" width="20%" onclick="machGroesser(this)">

<script type="text/javascript">
var w = 1;
function machGroesser(e) {
  if(w == 1) {
    e.removeAttribute("width");
    w=0;
  } else {
    e.addAttribute("width");
    e.width={option};
    w=1;
  }
} 
</script>

<img border="0" src="{param}" width="{option}" onclick="machGroesser(this)">

function machGroesser(e) {
 if(w == 1) {
   e.removeAttribute("width");
   w=0;
 } else {
   e.setAttribute("width", "{option}");
   w=1;
 }
}