title: How to show different size images on a site with lots of images
body: <p>For an image intensive size like 500px, flickr, instagram etc which serve lots of images with different sizes and users constantly upload images whats the best way to store and display different size images.
  Same image is usually displayed in different sizes, like thumbnail, modal window size and big size etc (like on ecommerce sites).
  Also for dynamic size grids like <a href="http://500px.com/flow" rel="nofollow">http://500px.com/flow</a> where images are all dynamic sizes spanning multiple rows and columns.</p>
  
  <p>Do these sites create all these different sizes and store different version of site or its done on the fly? </p>