Why Is My Picture Thumbnail Not Showing
Does the image open and appear in Preview.app? If not, the Finder will also not be able to open and create a thumbnail.

Show Icon Preview

Assuming the file is a valid image and does open in Preview.app, make sure Show icon preview is enabled for the Finder's view:


Select the file in the Finder
Select Finder > View (menu) > Show View Options
Enable the Show icon preview checkbox: