Instead of parsing day, month and year you can specify date formats directly using datepicker's formatDate function. In my example I am using "yy-mm-dd", but you can use any format of your choice.
$("#datepicker").datepicker({
dateFormat: 'yy-mm-dd',
inline: true,
minDate: new Date(2010, 1 - 1, 1),
maxDate: new Date(2010, 12 - 1, 31),
altField: '#datepicker_value',
onSelect: function(){
var fullDate = $.datepicker.formatDate("yy-mm-dd", $(this).datepicker('getDate'));
var str_output = "<h1><center><img src=\"/images/a" + fullDate +".png\"></center></h1><br/><br>";
$('#page_output').html(str_output);
}
});