I realized that when I rotate the image I imported to stage (a png image without background), Image gets somehow corrupted, what I mean is on the edges of the image there is anti-aliasing problem.

This occurs when I resize the image; Not expanding but shrinking leads that problem as well.

How can I overcome this problem ?

Below the example;
This butterfly is a png image I imported

http://www.fastswf.com/swfs/cM37zos/z5vAdo0wI8JtBw