I stumbled over a really minor issue: In the course list (the version with the bigger thumbnails) cover images are downscaled by 1px and therefore appearing slightly blurred (original image width: 180px, downscaled to 179px). This happens because the box-sizing-model is set to border-box and therefore includes the right 1px border. The fix is as simple as that: set the box-sizing for .o_visual to content-box so the border is excluded and the image displayed in its full and shiny 180 pixels. See the attachment for clarification.
(Tested with Chrome/OSX)