Icon Tip for NetBeans Platform

Then set the default icon as the "iconBase" in the action registration in the layer, the line is line 7 here:

<folder name="Actions">
    <folder name="Tools">
        <file name="org-demo-mod4-SomeAction.instance">
            <attr name="SystemFileSystem.localizingBundle" stringvalue="org.demo.mod4.Bundle"/>
            <attr name="delegate" newvalue="org.demo.mod4.SomeAction"/>
            <attr name="displayName" bundlevalue="org.demo.mod4.Bundle#CTL_SomeAction"/>
            <attr name="iconBase" stringvalue="org/demo/mod4/demoIcon.png"/>
            <attr name="instanceCreate" methodvalue="org.openide.awt.Actions.alwaysEnabled"/>
            <attr name="noIconInMenu" boolvalue="false"/>
        </file>
    </folder>
</folder>

Now, when the action is large/disabled/pressed/rolled-over, the relevant icon is shown. The important factor is that the "iconBase" is really the "base", with the others ("24", "_disabled", "_pressed", and "_rollover") being added on to the name of the base icon.