Add This CSS to your HTML in a STYLE tag or better yet put it in a .css file and reference it in a Link tag:
.saveBtn
{
background: url('/img/disk.png');
background-position:center;
background-repeat:no-repeat;
}
Here is the HTML input object. The CSS references it by the class name 'saveBtn'
<input type=button id="btnSave" class="saveBtn" value="Save"/>