<img src="g1a.png" id="modele" style="display: none;"></img>
<script>
var source = document.getElementById("modele"), rw = source.naturalWidth, rh = source.naturalHeight;
    if (rw / rh >= ($(window).width()) / ($(window).height() - 60))
{
    var param = ' width="' + window_width + '" ';
}
else {
    var param = ' height="' + window_height + '" ';
}
var window_height = $(window).height() - 60;
var window_width = $(window).width();
$('#pic').html('<img src="g1a.png" width="' + window_width + '" height="' + window_height + '"></img>');
$(function()
{
    $('#sel_div').css('top', 30 - $('#sel').height() / 2);
    $('#sel').change(function()
    {
        var selected = $(this).val();
        $('#pic').html('<img src="' + selected + '.png" ' + param + '></img>');
    });
});