The MediaContentSelectorView in Sitefinity shows thumbnails as squares. You can see what I mean in the designer of the Image control in the toolbox. The reason they don't show up in your case is a missing class, which has to be supplied to the MediaContentSelectorView. Here's the modified template, which should work fine:
If you don't want your thumbnails as squares, you have to include your own CSS styles and override the default ones by setting the appropriate width and height.
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