Do you mean icons on components palette, don't you?
Yo can take a look at plug-ins for spring JSF components here https://anonsvn.jboss.org/repos/jbosstools/trunk/vpe/plugins/org.jboss.tools.vpe.spring or more complex example for Richfaces JSF Components here https://anonsvn.jboss.org/repos/jbosstools/trunk/jsf/plugins/org.jboss.tools.jsf.text.ext.richfaces. They should be pretty explanatory.
I guess they might be added through extension point, but I cannot tell now for sure. I'll check this out and answer later.