CSS class and changing with javascript

I'd like to change CSS class values using javascript, what works?

html
<a href=http://www.sugababes.com><img
src="http://www.peepo.com/copyright/bands/sugababes.jpg" class="myLong"
/></a>

css
.myLong  {width:30%;}

js
myLong.style.width=10%

except this js doesn't work.

what would?

thanks

Jonathan