Hi Andreas Mahr,
You could set the LeftImageSource of the TreeViewAdv control from CodeBehind and we have attached a sample under the following location which illustrates this.
http://www.syncfusion.com/uploads/redirect.aspx?&team=support&file=TreeViewAdvSample1350844224.zipPlease let us know if you have any queries.
Thanks,
Mohana Sundari.N