Custom Control Source Property - Required Attributes

I have a custom control that derives from Button that has a property, Source
of type ImageSource.

However, when I'm in Blend, I do not get the listing of images in the
project like I do when I'm assigning a source to a built in Image control.

I've authored many controls in ASP.NET and know that the property probably
requires some attribute(s) so that Blend will display the list of images in
the project.

Does anyone know which attributes need to be applied to my property?

Thank you!

Karl