This is a simple solution but the question as been asked
This tutorial will teach you how to change the default avatar on Wowonder
Open your root files find and replace
For users
upload/photos/d-avatar.jpg for male
and
/upload/photos/f-avatar.jpg for female
For pages
upload/photos/d-page.jpg
For Covers
/upload/photos/d-cover.jpg
For Groups
/upload/photos/d-group.jpg
That is all. Have fun editing you site. :D