What is this picture? Even after referring to several books I simply cannot understand what this picture is depicting. I am not a Buddhist and am hence finding it difficult to decipher.



I found it on this page, whose title is, "The Buddha of Purification with consort". The description on that page says it depicts a manifestation of 


  "unity of the fully developed masculine and feminine energy: the complete purity and highest state of enlightenment".


If that's an accurate description, could you provide the explanation of that phrase?

The Buddha himself didn't want to get entangled in samsara, let alone unite with the other sex; so isn't this contradictory?
That would be yab-yum.  I think the wikipedia entry will more than answer your question.  You can find that here.  The one thing the entry only hints at, however, is that this is an actual practice in Tibetan Tantra.  In fact, it's generally what comes to mind, for a popular audience anyway, when the word "tantra" is used.  I think it's an exercise in futility to try and justify this practice according to Theravada or Mahayana ethics.  Within a tantric context, it just is what it is.