$(window).on('load',function(){
$('#myModal').modal('show');
});

<div id="myModal" class="modal fade" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
<div class="modal-dialog">
<div class="modal-content">
<div class="modal-body">
<img src="//placehold.it/1000x600" class="img-responsive">
</div>
</div>
</div>
</div>