ryoryo, I was having the same issue when I tried to import characters from Corel Paint and Draw into Flash. I fixed it by going back to my original image, and turning the object marquee on. (I'm not sure if this is the same in photoshop) essentially, its just a border that tells you where all the pixels are in that image, even those hard to see pixels.

From there you should be able to notice that the border is actually one or two pixels off of your sprite. This is why you get that faded white trim.

Remove those hard to see pixels and the faded white trim should be gone when you import to flash again.

I hope this helped!

Thank you so much for posting this.  It's been bugging me for a while, but this really helped. 

 this makes the files size go really high. Which in fact takes away from the whole idea of flash. Isn't it? but idea works. That's what I do to. Is there any other way that one could think of to get the transparency on stage as well as the files size kept lowest?

hey,

If you are exporting a transparent image inside photoshop, and you get those faint annoying white outlines when you import your images into flash, here is how to get rid of them:

1. Make sure your image has a transparent background (white and gray checkered background)

2. Go to  "help" on the top menu bar--then select "exporting a transparent image". After you do this, then another box will pop up. It will then trace your image (like what happens in Corel.) Then select GIF or PNG--the difference in these are the support from old and new browsers. For safe use for older browsers-select GIF. If your audience are people who predominately use newer browse, then select PNG if you wish.

Then, follow the steps it gives you, and name your file what you want it to be, and VIOLA! you destroyed the evil white lines.

I found a better way of importing Photoshop files into Flash. Must read, this is a very nice trick

http://www.Flashsourcecodes.Com/2009/11/26/how-to-import-a-photoshop-psd-file-into-flash-cs4the-right-way-to-allow-update-tutorial/