There is no simple route through user settings to do this.
You will need to provide an image file of your own and edit the NAML code in the macros that generate the icons you wish to change.
For example the code that generates the "People" icon on Nabble Support looks like this:
<macro name="people_action_link" requires="node_page">
<n.if.page_node.has_people_page>
<then>
<div class="[n.action_link_style/]" style="margin-left:.6em">
<img src="/images/people_sm.png" class="image16"/>
<n.page_node.people_link/>
</div>
</then>
</n.if.page_node.has_people_page>
</macro>
You would need to edit line 5 in the macro to provide the link to your image.
I provide some further clues on the process at:
http://greghelp.991552.n3.nabble.com/Add-Your-Own-Favicon-td4012849.html
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