Using firebug I can see that the image icon for the basicContainerNode has the class "dijitNoIcon" added to it, which sets "display:none" for the icon image. Whereas the leaf node does not have that dijiNoIcon class added. (I might need to look into that further as a possible defect)
But, as a workaround you could use some custom CSS to override what dijitNoIcon is doing.
<xe:basicContainerNode image="/vwicn148.gif" label="Container" styleClass="showIcon">
And add a custom css file to your application with the following:
.showIcon .dijitNoIcon{
display: block;
}