It took a while to finally solve this problem. Normally the location of the toolbar and tree icons is buried in a folder aspnet_client under the root. The problem is there can be several sub folders and it was difficult to determine which one to use. The solution was actually simple. Make a new folder named ReportImages then two sun folders toolbar and tree. Copy the respective icons into each folder. The add the following likes to the report aspx file

ToolbarImagesFolderUrl="/ReportImages/toolbar"
GroupTreeImagesFolderUrl="/ReportImages/tree"

Be sure to delete the folder aspnet_client locally and on the web server.