Ну вот собственно!
Форма регистрации пользователей, находим
Code
<tr id="siM64"><td class="manTdBut" colspan="2" align="center" id="siM65"><input class="manFlSbm" id="siF20" type="submit" name="sbm" value="$SUBMIT_SIGN$" /></td></tr>
И меняем на
Code
<tr id="siM64"><td class="manTdBut" colspan="2" align="center" id="siM65"><input class="manFlSbm" id="siF20" type="image" src="http://forucoz.com/img/button.png" name="sbm" value="$SUBMIT_SIGN$" /></td></tr>