The pointblank question is which Poser version are you using?  Before dispensing the correct advice we need to address which application.  Any version not labeled as Pro or Poser 10 will not have gamma correction as a render option, therefore you will not find any features to set in the image map node.  

If the hair piece is found under figures then it is a conforming wig which should be used for the character it was created for.  Unfortunately, if there is no cross fit available, you will need to manually manipulate the hair to the head and parent it so it follow the head when posing.

Now it the texture appears blurry, you either set the filtering to crisp if you are using the most current Poser version or none for older.