SPSite  _site =  new   SPSite ( " http://portal " );
SPWeb web = _site.AllWebs[0];
Microsoft.SharePoint.SPList list = web.SiteUserInfoList;
Microsoft.SharePoint.SPUser user = web.SiteUsers["DOMAIN\\portal.user1"];
Microsoft.SharePoint.SPListItem item = list.Items.GetItemById(user.ID);
item["Picture"] ="http://portal/SiteCollectionImages/portaluser1.jpg";
item.Update();