It took me a while to figure out how to modify or add a user picture from inside a custom module. Google was not much help. So here's the answer I found, hope it helps someone.
First, you need an actual Drupal file object. Something you could reference a file id from. Like $newbieFile->fid.
I referenced modules/user/user.module and includes/file.inc to figure this out.
$photo ="the actual image data."; //in my case it was the value of jpegPhoto from an ldap directory. //Save the pic to a temporary file. $writePic = file_save_data($photo); // Now $writePic contains the file stdObj.