How did you export this image from Poser? More specifically, where did the white background come from?

I suspect that you did not render over black, so the Poser background colour bleeds through into the semi-transparent pixels in the hair, giving the grey pixels that you see there. You can fix it like this:

Use the render setting "Render over: Black". Export the render as a PNG image, which maintains transparency. Then you can put your image onto whatever background you want and you won't have the grey mixed in with the semi-transparent pixels.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)