How do I get javascript to work in BBcode?

[Lizard King]

Try this

<script type="text/javascript">
function largerThan(a,b)
{
return a>b;
}
</script> 
 
<img src="{param}" onload="if(largerThan(this.width,500)){{this.height=this.height*500/this.width}{this.width=500}};" />