Thank you for the hint. I’ll revisit trying that when I can spare some time. I vaguely recall that doing so interfered with the way I ensured the consistent passepartout around my photographs while being responsive, but then again we are talking about CSS here and I just may have gotten things the wrong way around.
As a secondary tip: I believe you need to make sure that one of width or height is set to “auto” (in CSS). There was a good post on this on either CSS Tricks or SmashingMagazine. If I get some time I’ll try to find it and link it.