Hi,
you can change the
document type icon image files as described at the very bottom of
this document.
But if you want to have different icons for different nodes, this will require customization of the content tree code file (\CMSModules\Content\CMSDesk\Tree.aspx.cs) where you will check the drop down list value for given node and add some flag to it as you need to.
Best regards,
Juraj Ondrus