Repro Steps:

1) Create a Page
2) Introduce either an Image, or a NineSliceImage Element to the Page.
3) Navigate to its Properties Panel, and select any image as the ImageSource.
4) On the ImageSource Property, either MiddleMouseClick the Property, or from the 3-Dot Menu, select "Clear Local Value"

Observe how the blue Frame around the Binding disappears, representing a clearing of a Local Value.
However, Observe how the ImageSource URI is still present in the field.

Expected Results: The dropddown is able to support a 'None' value. (Similar to how a Path's 'Data' adopts a 'None' when cleared)