Thank you for writing.
This is an interesting feature that you request. The gif image does not provide any means of stopping the animation, because it has no play/stop functionality (it loops forever). The stopping and starting can be accomplished at a fairly low level by using the graphics object that paints the tree nodes. Our tree view however does not have such functionality. Please excuse us if this causes you an inconvenience - the tree view was not designed with such a feature in mind.
Here is a suggestion that may help in your case. You can change the images, based on the node's state. In a collapsed node you can have a static image, which resembles the GIF. Once the node is expanded, you can change the image to the GIF, which will loop until the node is closed.
Please write back if you have other questions.
the Telerik team