add some line-art to the list of images

[hrj]

Some references: https://commons.wikimedia.org/wiki/File:Bee_-_queen_(PSF).png

https://commons.wikimedia.org/wiki/File:Op-art-4-sided-spiral-tunnel-6.svg

http://duelistabbeyryou.deviantart.com/art/Brain-Spill-Uncolored-74995356

http://edelart.deviantart.com/art/Rooster-Peleador-175223593

[jonsneyers]

Maybe (scaled-down or cropped versions of) some of these would be interesting:
https://github.com/FLIF-hub/benchmarks/tree/master/test_images/PSF-line-drawings
https://github.com/FLIF-hub/benchmarks/tree/master/test_images/SMBC

I think FLIF would be pretty good in comparison with JPEG and PNG on images like those.
Also colored line art is more interesting for our purposes than black&white in my opinion, because single-channel (greyscale) images don't let you see the difference between RGB (png), YIQ (flif), and YCbCr (jpeg).