Well that all depends. If you're looking for someone who will show up in a Google search (a celebrity), then you Google search their name and then click on "images". If you are looking for someone who is not so famous that's more tricky. If you know them on a social network (e.g. myspace or facebook) you can grab an image from their profile.

Or, you could just ask them for a picture if you know them.

And I should say that if you're speaking of missing people, is this person already reported missing and their picture has been submitted. There are databases for that, hopefully this link shows the Google results:
Missing Person Databases

Thank you for your time and consideration