The random userpic selection when posting new entries or comments is caused by the FireFox extension 'LiveJournal Hook'. In order to resolve this issue, you'll need to either adjust the settings within LiveJournal Hook or uninstall the extension. LiveJournal Support volunteers cannot provide support for programs outside the LiveJournal service, such as LiveJournal Hook, so you'll need to consult LiveJournal Hook's help resources for information on changing its settings or uninstalling it.