First of all, you need to right click on the control you want to customize and select "Advanced -> Customize" from the popup menu, this brings up a frontpanel from which you can import picutres into the control (Edit -> Import Picture from file). Once the picture is imported, right click on the control and select "Import Picture", different control has different selection, for example, if the control is a boolean type, then you need choose a state for the picture (True, False and Decal). Import more pictures for all the states. When finished, save the control so it can be used later.
Search for the application note: LabVIEW Custom Controls, Indicators, and Type Definitions, for more detailed help.
-Joe
