View source of http://oyunzat.com and check the css class .qa-nav-main-link.
.qa-nav-main-link class was defined like background: url('images/question.png') no-repeat scroll 20px 8px transparent;.
You can find it and edit .qa-nav-main-link class in qa-theme/THEME/ folder.
Ex) For Snow theme:
qa-theme/Snow/qa-styles.css
Cheers,