In Opera Mail, see "Menu -> help -> about Opera Mail". It'll show you the location of the "mail" folder. accounts.ini is in there.
What you're talking about though is explained at http://www.opera.com/help/tutorials/mail/advanced/#custom-reply. But, all that allows you to do is to add plain text before the quoted text to fake a plain text signature above the quoted text. You can't do HTML there and you can't reference the signature set in the account's settings to have it placed above the quoted text.
Opera doesn't have a signature position setting at all.