Bob
sorry for the late reply - I just found this thread. DId you resolve this?
What I believe will work is
1) build footprints so the image borders are clipped off when zoomed into (nearly) full resolution, then
2) rebuild all overviews (the reduced resolution views)
What you're seeing when you zoom out are overviews based on the current view when they're built.
But one note - you said you built footprints with minimum value of 75 - this should normally be 0, or near zero if you have imagery using lossy compression (JPG, JP2, MrSID etc.). If you use 75, I'd expect a lot of darker pixels (values below 75) around the edge of the images would get clipped out by the footprint, contrary to your intent...
Let us know if this is still an issue
Cody B