Reduce thumbnails DPI

Created by: aubincleme

Especially on this page : https://learn.atilla.org/talks.html, about 8 thumbnails are loaded, each having a 1920x1080 pixels resolution. Each image then weights about 1.2 to 1.5M, which is quite a lot considering the fact that those images are just thumbnails.

This page weights then about 10M, and can degrade the browsing experience of a user far away from our servers (eg : in America) ; moreover, some people may sill have a low-data phone plan, and a 10M page could drastically reduce the amount of remaining data.

TO DO (proposed) :

• Reduce the size of each thumbnail to a more "web compliant" resolution (eg : 854×480 pixels ; https://upload.wikimedia.org/wikipedia/commons/e/e5/Vector_Video_Standards2.svg).
• If the full-resolution image is still needed somewhere else, upload separate thumbnails.