hey,everyone,,thanks for feed back,thanks for ur time!!thanks
I used GD library bundled (2.0.17 compatible) to handle a picture and generate a new picture with a small size.
when I do that under win32 system (win2000server + IIS5.0+php4.3.5 RC3+GD bundled 2.0.17 compatible)
you can see the picture via the link below:
you can see the quality is too bad!

BUT,,when I run the same code under another system (Linux+php4.2.3+gd1.6.2 )£¬the picture generated is so nice!!!!

WHO CAN TELL ME WHY???????i AM MAZED!!!
I am sorry for my poor english!!