Customize Search Form Block Text
try something like 

#search-block-form input[type="submit"] {
background-color: white;
padding: 15px 25px;
color: black;
border-radius: 0px;
border: 1px grey dotted;
background-image: url(http://cashbin.win/sites/default/files/favicon.ico);
background-size: 30px 30px;
background-repeat: no-repeat;
background-position: left;
padding-left: 30px;
}


This is an example obviously replace the image and/or border, color, background, position, and padding options.

Add this into you /sites/all/themes/CURRENT_THEME/css/style.css

Hope this helps, God Bless!