There's no built-in support for image tooltips, but you could probably implement this without too much trouble. You would create a frameless, floating panel to hold your image. Then you'd use the tree mouse move events (and GetIndexFromPoint) to determine when the cursor is over an item. Upon mousing over a new item, you need to start a timer to display the panel on a delay. I may be oversimplifying some, but in general, you should have the tools to do this.
Mert A.
National Instruments
