[blaze4218]

You cannot access a className this way.
If you used an id instead

#content
{
	height: 600px;
	width: auto;
	padding: 0px;
	background-image: url("images/kontor-1.png");
	background-repeat: no-repeat;
	background-position: center;
}

document.getElementById('content').style.background = 'url(images/kontor-' + currentIndex + '.png)';

but as long as you are using jQuery, you would't have to switch to an id...