How to change all Folder Icons to Show the content/Preview Image?
This cannot be done automatically across the computer, but there is a way to do this quickly to a group of directories:


Select the files/folders you want to change the icon of
Open the Inspector (Right Click -> Get Info) or Command + I
Drag a new icon


Using inspector will be best because you can select multiple files to change the icons of at once.

Also, as stated in this link, Apple will display file previews for some items by default.

Dynamic Previews Documentation: https://support.apple.com/en-us/HT201067

How to change file icons, as described above: https://support.apple.com/kb/PH13922?locale=en_US