Google even provided some tips. In your entry, include the mementos, the little things and the personal items that mean much to you. Google does not want you to focus on branding. They want to see a picture about you and your story. And if you don't own an Android handset, you can still enter anyway.
To qualify for the contest, you need to be at least 18 and a U.S. resident. The contest ends Tuesday, November 19th, at midnight. The picture must be in .jpg or.gif format and cannot exceed 2GB in size. An Android device (if you own one) should be in the center of the shot. There will be one entry accepted per person and the drawing to determine the winner will take place on Wednesday. One Nexus 7 (2013) will be given away along with seven $50 Google Play gift cards and winners will be notified two business days later. Good luck.
source: Google via AndroidCentral