PNG is a lossy compression. So the jaggys may also be the result of anti-aliasing.

Try exporting to .AVI not quick time. Or a series of .BMPs then join these to make an .avi.

I find trying to get Poser output rendered at the exact same resolution as mini DV, never  seems to work either. you have to enlarge or decrease your poser clip to make it work.  

Playing with some test chromakey pieces usualy gives me to get the best results to find the right resizing value.  

Pinky - you left the lens cap of your mind on again.