The difficult part of what you want to do is running your SubVI even if the value of the ring doesn't change. Without that requirement, I would suggest watching for the "Value Change" event on the picture ring and running your SubVI in that event handling case.
I think what I would try instead is use your own dialog for changing the value of the pict ring. Disable the ring control and watch for the mosue down event on it. When this occurs, call your own SubVI that runs modal. It will present an array of images to the user which they can click on one (or you can make the selection behave however you want). The SubVI will then return the value of the image selected, which you can use to update the value of the pict ring on the main VI.
J
Jason King
LabVIEW R&D
National Instruments