title: How can a virus exist in an image?
body: <p>I have recently watched  <a href="http://www.ted.com/talks/mikko_hypponen_fighting_viruses_defending_the_net.html" rel="noreferrer">this</a> video of a Finnish internet security expert. Somewhere around eleventh minute, he talks about a virus which is hidden in an image and executes when the image is about to be displayed.</p>
  
  <p>I am wondering how do they technically do such a thing, I mean how come the <strong>virus</strong> is <strong>executed</strong>, when the picture should be <strong>displayed</strong> and how come the picture is not compromised in some way. I thought the computer first looks at the extension, then opens it with appropriate program and lets the program work itself (and I don't expect regular image viewer to be able to run a virus within itself). Obviously it doesn't work like that, but no one I asked could help me out with this.</p>
  
  <p>So does anyone know how do they do this, the principle? Thank you very much.</p>