I think, personally, this UI fits in really nicely with Review Board's pre-existing metaphors. Great job!
A few notes:
We probably still want the filename in the center of this, rather than pushed to the right.
The labels should be vertical-center aligned
Either both should end with : or not.
We probably want a little extra space between "Custom" and the image on its left - maybe the same amount of distance between the background color selectors.
