Icon preview in Get Info shows file extension, not picture
Icon preview shows file extension after adding a pic to "get info"
Only shows the extension not the picture
the image was created or saved without a preview thumbnail icon
photoshop Save for Web will strip the thumbnail like this to reduce file size
or you can click on it and delete a thumbnail
(or paste one in) from your screenshot view -- this is how you add a custom image to a folder, one example
if your desktop or folders are showing such generic icons -- open the folder or click on the DeskTop -- then Command+J and check "Show Icon Preview"...