SetToolbarItemState() sets the pressed/depressed state of your toolbar icon. That is the only thing you can control in regards of mouse events
on the toolbar and typically is called from
void myplugin_pi::OnToolbarToolCallback ( int id )
m_iconpressed = !m_iconpressed;
SetToolbarItemState( id, m_iconpressed );
To receive this callback, you must return WANTS_TOOLBAR_CALLBACK from myplugin_pi::Init()
The ability to define the hover state icon was disabled in the core
when the UI styles were introduced and currently the toolbar generates the icon representaion from the first bitmap parameter of InsertPlugInTool() - the API is unchanged for backward compatibility.
You don't need SetToolbarToolBitmaps at all unless you want to change your icons based on the state of the plugin at runtime.
If you need more help, push your code to github or similar place.