In the view pane right mouse click on any of the Column Headers (i.e. FROM, SUBJECT,...) and select FIELD CHOOSER.
From the pull down menu select "Frequently-used fields" and look for the column header "icon". Then just click and drag it to where you want it positioned in the pane.
If you don't see "icon" in this menu then try "All Mail Fields"
Close the Field Chooser window when finished.