I'm having trouble getting the getElementById() command working properly, I've tested in various browsers and looked around to see if I can find the issue elsewhere and compared my code with others and I can't see a reason why it wouldn't work...

Here's the code in my header:
function header(x,y)
	document.getElementByID(x).style.display = 'block';
	document.getElementByID(y).style.display = 'none';
And the HTML:
HTML Code:
<form action="blah.html" method="post" enctype="multipart/form-data" name="adevent2">
   <input type="radio" name="heads" value="image" checked="checked" onselect="header('usefile','usecolour')" />Use background image<br /><br />
   <div id="usefile" style="display:block;">
      <label for="file">File to upload:</label>
      <input id="file" type="file" name="file" /><br />
   <input type="radio" name="heads" value="colour" onselect="header('usecolour','usefile')" />Do not use background image<br /><br />
   <div id="usecolour" style="display:none;">
      <input type="text" id="headbgsample" name="headbgsample" size="1"  value=""><br />