I did it other way.
this is how I add a class:
<p align="left">[submit class:buttons_mainpage " "]</p>
and then in the theme class I add a new class buttons_mainpage:
.buttons_mainpage {
background-image: url("images/but_submit.png");
background-repeat: no-repeat;
margin: 0px 190px 0px 0px;
padding: 0px;
float: right;
height: 26px;
width: 69px;
border: 0 none;
cursor: pointer;
}
Something like that ??