Installation Assistant: add logos to OS for faster clicking

IMO, to make things easier would be to add logos of proposed operating systems on the list where i choose from which os i am installing things, that makes it much faster in selecting where to click, currently here: https://tails.boum.org/install/os/index.en.html

(I actually vote for putting this list on the bottom of https://tails.boum.org/install/ instead of having a second screen.)

tentatively assigning to sajolida, with <3.

Original created by @u on 11341 (Redmine)