Pruebalo de este modo.
Código HTML:
Ver originalfunction f_mostrar(){
input = document.getElementById('p3');
input.style.display = "block";
}
<input type="text" name="p3[]" id="p3" value="Hola" size="3" style="display:none" />
<img src="http://www.altracocina.com/wp-content/themes/this/images/logo.jpg" onclick="f_mostrar()" />