This is perhaps related to http://drupal.org/node/937562
The error there is no longer present - setting a default user image does not break the site.
However - the user pic is not displayed on post or comment.
Installed a clean 7.0-beta2. Added the same default pic in the account config as in http://drupal.org/node/937562 - created an article and commented. No pic displayed.
Comments
Comment #1
pillarsdotnet commentedThis patch fixes it for me: http://drupal.org/node/935592#comment-3861338
Comment #2
pillarsdotnet commentedThis bug has been fixed, according to http://drupal.org/node/935592#comment-4501198
If not, it is most likely a duplicate of #1028092: Default image is not set to permanent and saved to the wrong schema or #799178: User default picture path incorrect when run through image styles