Resized large images for the thumbnails when needed

Created by: Alterrien

This PR is not to be merged right now, I'm waiting for more feedback on how the different thumbnails look on different sized screens (if somebody has a 2k-plus monitor, he's welcome to give some feedback) I've resized the thumbnails to 854x480 using convert $file -resize 854x480 $newfile, which should give good enough results. Other solutions could be tried.

Cheers !

Resolves #59 (closed)