Check the source of the image online, it is likely from a copyrighted website in which case avoid it at all costs. If you are making a poster for putting up around town the chances are the image is not high enough resolution anyway.
If you have access to other images I'd use them, fan clubs or Flickr searches for non-copyright photos by fans. Publicity photos are often put out for use by promoters so as another poster suggested contact their publicist. If it is a small local singer who doesn't have all this stuff, I'd try and contact them directly and let them know you are trying to promote them and could do with some material to help you.
This is a tricky area as some artists welcome fan input and effort, where as other artist just want to sue anyone who doesn't pay for the privledge. You don't want to be on that side of the law believe me.