WOImage and CSS class

It may be a stupid rookie question but I am trying to add a CSS class  
to an image, something like:

<img src="img.gif" class="leftalign" />


When I take WOImage there is no opportunity to add something like a  
class attribute.
I also found the ERXImage class of the Project Wonder API which  
suports a cssClass attribute, but with ERXImage I have problems to  
display my images which I keep in the WebServerResources/img/  
directory of my Application. While WOImage works perfectly with the  
filename attribute, ERXImage needs a relativePath attribute which I  
am unable to create. Does someone know how to create a correct  
relativePath for ERXImage using the WO-Framework when the image file  
is e.g. WebServerResources/img/img.gif.
I tried this.application().resourceManager().urlForResourceNamed in  
the Main class but it failed.

Someone could give me a clue?

Regards,

Helmut