Tutorial images are not optimized

[florianeichincode]

Expected Behavior

The tutorial images under src/lib/libraries/decks/thumbnails should be optimized so that less bytes needed to load them

Actual Behavior

The tutoria page needs 3.67MB to load all images. This can be reduced drasticaly if better compression is used and maybe a logic is implemented to generate a webp or avi automaticaly if the browser supports it.
I had a coder dojo where the internet access is not so good an 12 notebooks where connected. So the page load time was not so good

Steps to Reproduce

Steps to reproduce the behavior:

1. Go to 'https://scratch.mit.edu/projects/editor'
2. Click on 'tutorials'
3. Look at the browser network tab

I can optimized the images if you need help for that.

Thank you for the good work with scratch