title: What do these numbers mean after png image path?
body: <p>I have seen following CSS rule at two website</p>
  
  <pre><code>/* First example */
  background: #F8F8F8 url('/images/noise.png?1333540701') top left;
  /* Second example from a different site */
  background: #F8F8F8 url('/images/noise.png?1326131369') top left; 
  </code></pre>
  
  <p>What does <code>*.png?randomNumbers</code> mean after png image path? What purpose they serve here?</p>
  
  <p>First one is from <a href="http://batsov.com/" rel="nofollow">bastsov</a> and the other is from <a href="http://mattgemmell.com/" rel="nofollow">Matt Gemmell</a>. Both use Octopress framework.</p>
  
  <p><strong>Edit</strong>
  It is not that both rules are being used at the same site. Each rule comes from a different site.</p>
  
  <p>Moreover, it seems they use the same number everywhere, whenever <code>noise.png</code> is mentioned. For example,</p>
  
  <pre><code>/* rule for navigation bar */
  background: url('/images/noise.png?1333540701'),linear-gradient(#E0E0E0,#CCC,#B0B0B0)
  /* rule for #main */
  background: #F2F2F2 url('/images/noise.png?1333540701') top left;
  </code></pre>
  
  <p>I observed the same pattern but with different numbers at <a href="http://mattgemmell.com/" rel="nofollow">Matt Gemmell</a>'s site. </p>