see if this topic helps you
http://community.elgg.org/discussion/view/677375/tidypics-photos-missing 

The files are created and owner by your web server. If you changed the permissions such that that web server can no longer read them, the images will disappear. Good default permissions are 755 for directories and 644 for files where the owner is the web server. There is information on our wiki that explains how unix permissions work. Other than that, we would need to know more information about your accounts to help (what account/group is the web server running under, what are the permissions on the files, etc.)