Finding My Source Code

Hello,

I use WordPress version 3.6. I noticed someone has gotten into my site and posted a long ad for prescription drugs behind my header picture. When I am logged into WordPress it does not appear but if I am logged out and go through Google or Mozilla I can see it. A friend said I can delete it from my source code but for the life of me I can not find my source code. Through Appearance, Editor I was able to find these codes only:
Stylesheet
(style.css)
archive-dgx_gallery.php
Theme Functions
(functions.php)
sidebar-archive.php
single-dgx-forms.php
single-dgx_gallery.php

If anyone can help I will be forever grateful. If my boss sees it, I am finished.
Thank you,
Mardee