The correct and web etiquette polite thing to do is to contact the site owner and ask permission. Assuming that the photo in question actually belongs to him and he didn't himself just go grab it somewhere, then honor is served.
The problem with going the honest route is that if the owner doesn't write back, then he is alerted that you want his photo. If it's his very favorite photo in the whole wide world, and you use it, then this might start a confrontation.
So, you could just out and out swipe it.
Be polite and write.
Use it with accreditation.
The only completely correct answer is to ask first. But whatever you do, be sure to download it. Never just point a link at it so you're using his server to serve the pic to your site. Now that is the worst and will for sure make any webmaster ill when they discover it.