I have added a custom action link in Document Actions. The link is working fine but I am not able to get the icon for the link.

I have placed ic_publish_48px.png in ../tomcat/shared/classes/alfresco/web-extension/.
In ../tomcat/shared/classes/alfresco/web-extension/share-config-custom.xml file I have added the icon as:
<action id="publish-to-pubs" icon="./ic_publish_48px.png" label="Publish">
How can I get the icon on screen?