DOC optimized PNG images with OptiPNG

Did "optipng -o 10" on all the files with OptiPNG 0.6.4.
This shaves a total of 529012 bytes off the pictures.
DOC remove qda and faces from website carousel

... in order to speed up the website, as discussed at #2477.
DOC replace jquery by minified version

Still 1.7.2, as I saw in the history that @jaquesgrobler had
previously downgraded to this version.
Merge pull request #2479 from larsmans/website-speed

[MRG] improve website speed, hopefully