We don't know anything about your PNGs, your footage interpretation and so on. Transparency fringes can have all sorts of causes.
^ Yeah, what Mylenium said.
A quick-fix solution would be to either use the Matte Choker or the Simple Choker (found in Effects & Pre-sets > Matte) and play with the settings to see if you can pull a pixel or two off the edge of your image.
We don't know anything about your PNGs, your footage interpretation and so on. Transparency fringes can have all sorts of causes. Mylenium
This might help, dunno. In the olden days, AE artists were forced to use graphic elements produced by Photoshop geeks who had no comprehension of video's technical specs, they only understood print. So they'd compose on white paper. The alpha channels they supplied usually carried a fringe of white.
Try changing the alpha interpretation. It looks to me like it's set to premultiply. try setting it to straight. Select the footage in the Project panel and then context click (right click) or go to File>Interpret Footage>Main and change the interpretation.
Well, thanks everyone for your answers. he thing is that i went to what Rock suggested but Interpret footage wasn't active so couldn´t hange anything there.
The next step, Photoshop and lke bogiesan said, i was using a png that cames from and image with white background, so switchng it to black background and then make the Png do make the trick. Thanks, this confirm that if one doesn´t have the answer(i coldn´t figure that was the origin background of the image), perhaps another do have the clue, so issue solved.