When there is not much space available (e.g. front page, portal pages), sometimes a cropped version of an image is used, linking to the full version:
This takes up little space and looks much better than scaling down the original image to the same size. (example)
MediaViewer currently does not load such images because it bails out on any image with a link parameter. Ideally, it should not bail out but load the target (non-cropped) image instead of the thumbnail.