Hello Robin Verhulst,
I noticed that your collection has a number of levels, therefore I'd suggest you use the GetItemByPath() method of the RadTreeView to get the radTreeViewItems and then you can set the DefaultImageSrc like you do.
I prepared a sample project demonstrating this approach. Please take a look at it and let me know if it helps. Also keep in mind that in order to use the GetItemByPath() method you will have to override the ToString() method of the business class.
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