[Change Request] Compress images if acceptable

[CunliangGeng]

Check #98 first.

The images can be compressed a lot without affecting its display effects, which will reduce the size of the repo and speedup a lot for webpage loading.

To do that, you could test how much you can save for your chapter using https://tinypng.com/analyzer,
then you can compress the images using https://tinypng.com/

Maybe you're not happy with the compressed images, it's OK to keep using original images for better display effects.

The following chapters have been checked/compressed:

• setup
• ch 1
• ch 2
• ch 3
• ch 4
• ch 5
• ch 6
• ch 7
• ch 8

[bartvanb]

My lessons only have 1 image which is ~28 kB. I don't think it's worth the effort to reduce that image size - feel free to reassign me if you disagree