Unbelievable, but I had to spend half an hour to find out this.
If you want to create your own ContentControl in Silverlight (for example, your custom Image control displaying a mirror), you can do it as follows:
1) Create a class inheriting from ContentControl.
2) In its ctor, write the following:
DefaultStyleKey = GetType();
3) Create the folder “Themes”, put the file generic.xaml there and insert the following content:
Now you can implement your control as usual, and it will get the XAML above as its ContentTemplate by default.