As a user of Windows primarily, I was stunned not to see this enabled globally by default, but whatever – that doesn’t matter. You can define thumbnails to show in individual folders or globally throughout all accessible folders with the following:
1. Open ‘Finder’ by clicking the Blue Smiley Face icon, this should be to the left/top of the ‘Dock’. If you want thumbnails to only show in specific folders, navigate to the folder using finder first using ‘Finder’, then complete the following steps…
2. In the menu now at the top of the desktop, select ‘view’, then ‘Show View Options’.
3. Select the ‘All Windows’ or ‘This Window Only’ (depending whether you want thumbnails globally or within the current folder only) radio button, then click the box beside ‘Show Icon Preview’.
4. You can now close the options window using the red ‘x’ at the top-left of the window.