Crop, resize and optimize shelan-tn.png

Crop, resize and optimize shelan-tn.png

The original image was 2055px × 1252px, which was oversized
and did not fit the required dimension of 600px × 400px
(3:2 aspect ratio).

To fix, the image was cropped and resized to the required dimension,
and was further optimized:

    $ pngquant --nofs -v --speed 1 --quality 65-80 shelan-tn.png
    $ optipng -o7 -zm1-9 shelan-tn-or8.png
    $ mv shelan-tn-or8.png shelan-tn.png

reducing its filesize from 334125 bytes to 26929 bytes.

See #1831