From LabVIEW Wiki
Setting the icons of an ActiveX tree view control
First create two ActiveX containers on the front panel. Place the TreeView in one, put an ImageList in the other. Second, right click on the ImageList and bring up the editing dialog by selecting either Property Browser or ImageList|Properties (I suggest the latter). Thirdly, use the editing dialog to paste in the images you want and set any other properties. Fourthly on the block diagram set the ImageList property of the TreeView to be the ImageList. Lastly, select the images by name or number for each Tree node.