Why does my sharp images get blur after I add my watermark?

Yes,  I see they both look blur now as well.  I'ts not like that on my Mac.  I save the water mark in PNG and the back ground is transparent.  When I have my edited picture, I move my water mark on top of the pic and free transform to position where I want it.  I SAVE AS my finished pic to my desktop from CS5.  I then move it back into aperture.  When I compare the original edited file then see the file with the water mark there is a big difference on my MAC.  Here they both look the same (slight blur) but the original is sharp.