If you are set using Bootstrap 2, then your best bet is just to do what the question you referenced says, and add a specific rule in your CSS, as the 2.x branch does not have anything specific for responsive images.
Another option to consider would be to move to Bootstrap 3, which does have a responsive image helper. It is as simple as adding .img-responsive
to any image. See the 3.x docs for more details at: http://getbootstrap.com/css/#overview-responsive-images.