如何在Expression Blend中访问自定义用户控件的UI元素

I am new to Expression Blend. I want to create a custom user control which consists of an image control. I want to be able to set different image source for each instance of the custom control. I am able to do this in code-behind, but I couldn't find a way to do this directly inside Expression Blend (like what can be done to the image control in property panel). The image control inside the custom control is not accessible in the visual tree. If I change the image source in the custom control xaml, it applies to all instances.I am new to Expression Blend. I want to create