fil transparent selection with another part of image

[Rhods85]

Hi

I need to edit an xray. I have created a shape from one section of the image which i can move around. I have moved the selection shape to another part of the x ray. I want to fill this selection shape with the colour/gradient of the image behind that it sits atop. How do i do this? (Mac OSX)

I've added an attachment of where i am at the moment.

Thanks!
Rhods85