Do I need to unlock boot loader to flash official factory image of Android L Preview ? And will it void warranty? Is it rooting the phone ? I have never rooted phone before and I don't wish to do so. Please provide useful link on how to flash such images.
I think in the USA it should not void your warranty as it is regulated by law and the manufacturer must prove you actively damaged your phone in the the process (which would be at least funny if you are putting a Google Android Image on a Nexus phone and following the steps Google itself published, for example).
The Magnuson-Moss Warranty Act states on Title 15 of US Code, Chapter 15 (Consumer Product Warranties), Section 2304 (c) that the manufacturer must prove the damage inflicted by the user to void the warranty, as explained on this video.
In the European Union there is an equivalent ruling on EU Directive 1999/44/CE