Silverlight Image in Listbox

I have a listbox with a data template. The problem is that it expects the source to be a string. The string I have is a uri of an image inside the xap file. So it would be uri( xxx, uri.relative) How do I get it to look inside the xap file for the image since I can only use a string value?

ListBox.ItemTemplate  
DataTemplate  
StackPanel Orientation=Horizontal VerticalAlignment=Center

Image Source="{Binding Path=Image}" Width="50" Height="50" Margin="0,0,10,0"  
StackPanel 
DataTemplate  
ListBox.ItemTemplate

//it won't let me use URI for the Image return value!!!

public class MyListboxItem

{

public String Image

{

get { return thumb; 

}

}

Silverlight Image in Listbox