If you're looking for just replacing the default picture for the control with a custom one, you can customize the picture for each state of the control. For the boolean example, there's 4 states: On, Off, In the process of being pushed but not on yet, and In the process of being depressed but not off yet. I've attached an example. I took a screen capture of my Noia extreme themed "OK" button in the on and off state, copied each and imported the pictures from the clipboard to replace the four default pictures of a simple boolean (use the classic ones to customize...they have less "stuff" to deal with). When you're in the customize "VI" for the control, click on the wrench and you'll enter well, I don't know what to call it, but it's the "Really Customize Mode" where you have access to all the graphics of all the states of the control and you can import from the clipboard or a file directly.
For the attached example, the background would obviously have to be white, but I haven't played around with different graphics types which might preserve the "transparent" color value of the background. There might be a way.
Hope this helped!