To ensure variety on the site I think we need both a "close up" and a distant shot, so please specify
Category 1a (Silbury as the main subject) or Category 1b (Silbury in the landscape)
Try this for 1b, showing the moat and taken in Feb from the top of Waden Hill:
http://www.themodernantiquarian.com/post/11002
This (again for 1b) shows the first site of Silbury, when approaching the Avebury landscape on the Ridgeway from the south, though the arrow is necessary:
http://www.themodernantiquarian.com/post/10010
This for 1a:
http://www.themodernantiquarian.com/post/10007
All can be provided in greater detail.