Thumbnail images are generally taken for granted -- until you have to make them for large quantities of photos. Resizing photos into tiny squares and saving a separate copy is far more time consuming than you would think. If you're on a Mac, you can use the default image viewer, Preview, to make thumbnails by using the "resize" function. However, if you're interested in a far less time-consuming method, Mac OS X also comes with Automator, which lets you create actions to automatically make thumbnails from your images.
Open your "Applications" folder on your Mac and double-click the "Automator" icon to open the application. Click "Action" from the startup screen.
Click the "Photos" link in the far left column of the application window. In the second column, click "Create Thumbnail Images," drag it into the workflow window on the right and drop it when you see the green "+" symbol.
Click the tab next to "Size" and choose a maximum pixel size for your thumbnails. Automator lets you create thumbnails with a longest edge of 72, 96 or 128 pixels.
Click the tab next to "Suffix" to apply a custom suffix to your thumbnails. Automator automatically applies the suffix "tn" for "thumbnail" to your photos, but you can change it to anything you like. Adding a suffix to your photos protects your original images by making a copy with a different name.
Select "File" from the top menu bar and then select "Save" to save your action application. Click a destination to save on your Mac, such as your Desktop, and type a name for your action, such as "Thumbnails" into the field. Click the "Save" button to save your application.
Locate and select the image or images from which you want to make thumbnails. Click the "Finder" icon in your Dock to create a new Finder window and locate your saved thumbnail Automator action.
Drag your images and drop them into the action icon to create your thumbnails. You can drop one or multiple images into the action, but if you choose multiple images it is important to allow the application to completely finish the thumbnail action before attempting to open or use both the original and the thumbnail images.
You can also create individual thumbnails using Preview, which is located in your "Applications" folder. Open an image in Preview and then select "Tools" and "Adjust Size" from the top menu bar. Type a desired size for your thumbnail, such as 100 pixels, into the "Width" or "Height" field. Click the box next to "Scale Proportionally" to constrain your image to the original proportions when resizing. Select "File" from the top menu bar and then select "Save As" to save your thumbnail as a new file that does not overwrite your original image.
Create a Dock icon for easy access to your most-used Automator actions. Drag the application or workflow icon for your action into the desired location in your Dock and release your mouse to create an icon. Drag photos or files and drop them onto your Automator application or workflow icon to run your action from the Dock.