Poor quality when zooming in/out

Hi!
Firstly, sorry for poor english.
I scale my jpg (progressive or normal), say 600X500 pix and then scale this picture down to 75% by a motion tweening to create a zoom out effect. The problem is that the whole picture gets poor quality during the motion. Some shrinking and noisy effects appear on the picture.
Can anybody help me on this problem ? :ne:

Thanks a lot.