The team blog of the Expression Blend and Design
Because Windows Presentation Foundation (WPF) is based on the .NET Framework, you can customize the functionality of existing control types. You can start with a control defined in a WPF library, derive a new control type from it, and build that into a library of your own. You can draw a control onto the design surface of Expression Interactive Designer just as easily whether it is implemented in your own library or in a WPF library. In this tutorial you will extend the Button control to create an ImageButton control which has a Source property in addition to the standard Content property. You can then specify how the new Source property is to be used by the control by modifying the control's template.