Make a new text document and paste this into it:
<html>
<head>
<title>RuneScape Desktop</title>
</head>
<body> bgcolor-"black">
<div align="center">
<iframe src="http://www.runescape.com" width="1100" Height="800"></iframe>
</div>
</body>
</html>
Save it as runescape.html
After that, right click your desktop, go to properties, background, browse, and select the file your just made. Hit ok; and there you go. Runescape on your desktop.
If you want to change the size, just change the numbers by "width" and "height"