предлагаю использовать ID и getElementById()
Код:
function DecCnt(ident){if (document.getElementById(ident).value>1) document.getElementById('delta').value--;}
function IncCnt(ident){document.getElementById(ident).value++; }
<a href="javascript:void(0);" onclick="DecCnt('delta');"><img src="pics/left_arrow.gif" width="4" height="7" alt="-"></a>
<input type="text" name="delta" id="delta" class="count_prod_box" value="1" id='count' maxlength='4'>
<a href="javascript:void(0);" onclick="IncCnt('delta');"><img src="pics/right_arrow.gif" width="4" height="7" alt="+"></a>