Prova così:
<div style="float:left;"><input name="invia" type="image" style="width: 79px; height: 21px; border: 0px; margin-top: 8px;" title = "Vai allo step seguente" onclick="this.form.action='path'; this.form.submit()" value="invia" src="buttonContinua.jpg" alt = "Vai allo step seguente" align="bottom" width = "79" height = "21"></div>
<input name="indietro" type="image" style="width: 72px; height: 21px; border: 0px; margin-top: 8px;" title = "Vai allo step precedente" onclick="this.form.action='path'; this.form.submit()" value="indietro" alt = "Vai allo step precedente" align="middle" width = "72" height = "21">