By default, Mac OS X shows a generic, white icon when displaying a thumbnail of an image in a folder or on the Mac’s desktop. Many people find this frustrating, especially switchers who are used to the way Windows shows a little preview thumbnail of the actual image.
Fortunately it’s very easy to change this, so you will see a thumbnail preview of your images. Whenever I get a new Mac, this is one of the very first preferences I set.
From the Finder’s menu bar at the top of the screen, click on View > Show View Options (activate the Finder by clicking on its blue “Happy Mac” icon at the end of your dock, or by simply clicking on an empty space on your Mac’s desktop). In the Finder, you can also use the keyboard shortcut Command-J.
Once the View Options menu opens, check the box that says “Show icon preview.”
Note that you can also choose the size of the icons in the View Options menu. Close the menu, and now all of your generic thumbnails will show a preview of the actual image.
Also, it may take a second or two for OS X to create thumbnails in folders with hundreds of images.