title: Writing complex custom metadata on images through python
body: <p>I'm looking to write custom metadata on to images(mostly jpegs, but could be others too). So far I haven't been able to do that through PIL preferably (I'm on centos 5 &amp; I couldn't get pyexiv installed)
  I understand that I can update some pre-defined tags, but I need to create custom fields/tags! Can that be done? </p>
  
  <p>This data would be created by users, so I wouldn't know what those tags are before hand or what they contain. I need to allow them to create tags/subtags &amp; then write data for them. For example, someone may want to create this metadata on a particular image :</p>
  
  <pre><code>Category : Human
  
  Physical :
      skin_type : smooth
      complexion : fair
      eye_color: blue
      beard: yes
      beard_color: brown
      age: mid
  
  Location :
      city: london
      terrain: grass
      buildings: old
  </code></pre>
  
  <p>I also found that upon saving a jpeg through the PIL JpegImagePlugin, all previous metadata is overwritten with new data that you don't get to edit? Is that a bug?</p>
  
  <p>Cheers,
  S</p>