The file that contains the icons for the Views is here: [MC program path]\Data\Default Art\SmallIcons.png
But that isn't part of the skin, and changing that is troublesome as updates to MC will overwrite the changes. Long ago you could make a Custom Art folder and apply the changes there, but I think that doesn't work anymore for SmallIcons.png (though I could be wrong on this).
In any case, the ones shown in my screenshots are stock, and not part of the skin at all.
You may not realize that each View in the Tree can have its own icon, and that these are also useful for sorting and organizing the Views in the Tree. Select any of your Views in the Tree and right-click on it and choose Customize View. Within the View's Settings Panel (on the right), you'll find: Tree > Image. You can choose from a variety of icons for each view individually. Note that these icons also sort together in the Tree and have a pre-defined sort order.