To all who need the same change:
in fact there are two files that should be edited:
components/com_phocaguestbook/views/phocaguestbook/tmpl/default.php
there I add:
class="contact-button" at line 577 to both buttons Submit and Reset
After that in file
components\com_phocaguestbook\assets\phocaguestbook.css
I add
#phocaguestbook .contact-button {
display: inline;
margin-top: 5px;
background-color:transparent;
background-image:url(/images/submit.png);
border:0px solid #CCCCCC;
font:12px Arial, Helvetica, sans-serif;
color:#ffffff;
width:100px;
height:26px;
}
And everything work fine
