Форма регистрации пользователей, находим
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<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 это:
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<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>
]
