Gravatar : Is there a default image?

To select a default image, you can use the d parameter:

http://www.gravatar.com/avatar/3b3be63a4c2a439b013787725dfce802?d=identicon

Or without a hash:

http://www.gravatar.com/avatar/?d=identicon

Or without the d parameter:

http://www.gravatar.com/avatar

Source: gravatar.com.