How to create a Chrome extension that adds code to the page

Question

Extension Safe or not:

I need to add in a specific page (preferably) or in all cases if a certain code is simpler.

This here works:

var meucode = 'alert("oi");';
var script = document.createElement('script');
var code = document.createTextNode('(function() {' + meucode + '})();');
script.appendChild(code);
(document.body || document.head).appendChild(script);

He gives an alert on every page I enter...

Well, it is possible to adapt this to add an "image" on the site that the person enters.