Convert all images form tiff to png/jpg

[mikepan]

All images are currently in tiff format (optimized for print), we need to covert to png or jpg for online viewing.

Sub tasks:

1. convert all tasks to png or jpg for chapter 3-10.
2. Update the screenshot when necessary to the newest Blender version.
3. Modify the .md file to link to the new files.
4. Optimize the layout