Saluton
You can localize buttons on monobook toolbar by adding code like this to MediaWiki:Monobook.js of your wiki.
if (wgAction == 'edit' || wgAction == 'submit') addOnloadHook(function(){ if (mwEditButtons.length < 2) return; // bold button mwEditButtons[0].imageFile = 'http://upload.wikimedia.org/wikipedia/ru/9/9a/Button_boldru.png'; // italic button mwEditButtons[1].imageFile = 'http://upload.wikimedia.org/wikipedia/ru/8/88/Button_italicru.png'; })