Would be simple enough using a plugin, a script, or just a bunch of curl commands, however I think it would be better to pressure Atlassian into supporting a gravatar-like service JRA-24653, as Fisheye already does.
Otherwise it's a constant pain to keep the pictures uptodate, and deal with the fact that users can upload their own picture which you probably would not want to overwrite, etc etc.
EDIT: I just saw you tagged confluence, but I believe my comments equally apply to jira and confluence.
Just to clarify your question...Do you want to upload the same picture to use for all users at the same time?
If you want ot implement a new feature in a future version of Confluence, please feel free to submit a feature request at http://jira.atlassian.com, and we will study the possibility of developing.
For more information on how Atlassian implements new features and improvement please see the following document: Implementation of New Features and Improvements: http://confluence.atlassian.com/display/DEV/Implementation+of+New+Features+Policy
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