[button] How to set an image via classname

Hi*
I'm trying to specify an icon outside the CSS framework.
with the markup
<a class="ui-button ui-button-costom.png" title="Aggiorna la
quotazione della pratica."
href="#" >
Aggiorna quotazione
</a>
I can not see the image "custom.png"
Can someone give me an example?
The image is not part of a CSS sprite but a single PNG.
How can I possibly specify a path with. and .. ?
Tkz a lot!
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