Assuming that you use bootstrap 3 (should work similar in 2 and 1), then, just include within the body of the modal an html tag of images 'img' and by javascript or php (I leave it to your choice) assign the route of the image in the property 'src', a simple example would be like:
<div id="myModal" class="modal fade" role="dialog">
<div class="modal-dialog">
<div class="modal-content">
<div class="modal-header">
<button type="button" class="close" data-dismiss="modal">×</button>
<h4 class="modal-title">Modal Header</h4> </div>
<div class="modal-body"><img src="http://static.batanga.com/sites/default/files/styles/full/public/universo-observable-en-una-imagen-3.png?itok=sBpiT7gx" class="img-rounded" alt="Cinque Terre" width="304" height="236" /> </div>
<div class="modal-footer">
<button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
</div>
</div>
</div>