White dots with ultra key

Hi,

I have been having some problems with rendering *I think it is on the rendering side* I am taking a 4k video and running a key on it and putting it in front of a picture. The picture (seen below) is about 6600x2800 and when I make the sequence from that photo and render it white dots appear around my keyed footage. But! When I make it from the 4k footage, and then shrink the photo to fit, and then shrink my talent to fit in the photo correctly (inside the letterbars) No more white dots!

But this means that I have to adjust for this for every cut, and quite frankly I should have to.

So is this a media encoder error? an Ultra key error? (NOTE: the playback is FINE before rendering) or an error somewhere in premiere? 

And any idea how to fix it?