Ci sono moltissimi modi di implementarlo, una possibilità è
<style type="text/css">
#search_form {
display:inline-block;
font: bold 15px Verdana,Geneva,Arial,Helvetica,sans-serif;
font-size:x-large;
color:black;
}
#search_form label {
padding : 0 15px;
}
#search_form input[type="image"] {
display:inline-block;
height:35px;
}
#search_input {
font-size:xx-large;
}
</style>
<form id="search_form" action="/..." method="post">
<label>Ricerca <input id="search_input" size="30" name="keyword" /></label>
<input type="image" src="/my_image/search.png" />
</form>