How do I edit this "GO" button in Magento?

I have a simple Magento website. It's still very basic cause I'm a beginner.

I tried but have failed badly. This is what I want to do: Remove the "GO" button (submit) and place a small image instead. this is my website: http://gale.dev4.webenabled.net/

any help? Please开发者_开发问答... I tried editting so many files but I can't get rid of this code:

<span><span>GO</span></span>

---

Add what you need to the end of styles.css

.header .form-search button.button span span {
    display:none;
}
.header .form-search button.button span {
    width:50px;
    background-color:red;
}

That will give you a red box, you should be able to take it from there.