There may be times when you have a photo or other image on your Mac that you would like to resize. These may have been downloaded from the internet, taken on your iOS device or with another camera, or taken as a screenshot on your Mac.

While often you can embed an image in a document or e-mail, and then resize it in this medium, sometimes a relatively large image can be cumbersome to deal with. For instance, a large photo you send to someone might be opened on his or her computer and end up being relatively large in size and fill the entire screen.

Even though most image-handling programs have options for displaying photos at smaller sizes within the program’s window, you can also resize the photo yourself and avoid having to deal with a program or two showing it at full scale.

Preview

When managing images on your Mac, your best bet for built-in options will be Apple’s Preview program. While simple, this program offers some quick ways to crop, resize, annotate, and otherwise edit images. This program is best for managing images you have organized directly in the Finder, as opposed to those in iPhoto or other programs that you have in a library, and can be used to resize images in a few simple steps:

1. Open the image in Preview.
2. Choose “Adjust Size…” from the Tools menu.
3. Select a preset, or choose Custom from the “Fit Into” menu.
4. Click “OK” to apply your settings.

Automator

If you regularly change the size of images you have organized in the Finder, then you might benefit from creating an Automator plug-in to do so directly in the Finder, instead of having to open the files in Preview. This is especially true if you are simply changing the dimensions of an image, and are not interested in the image’s DPI or other features:

1. Create a new Service Automator workflow
2. Choose “image files” as the type of files the service receives (in the menu at the top of the workflow). You can leave the relevant application as “any application.”
3. Add the “Scale Images” action to the workflow, and choose “Don’t Add” for the offer to include a file-copying action.
4. Set the preferred settings for this action (e.g., use “By Percentage” at a value of 50).
5. Click the action’s Options and check the one to show this action when the workflow runs.

Save the workflow with a name such as “Resize Images” and then you can select a number of images in the Finder, right-click them and choose this new service from the Services contextual submenu. You can also choose this from the Services section of the application menu (the one to the immediate right of the Apple menu), provided the application can pass image information to this service.

Lastly, with this service in place you can go to the Shortcuts section of the Keyboard system preferences, and then click Services, followed by locating the new service in the right-hand list. In here you can add a custom hotkey shortcut that can be used to quickly apply this service to any selection of images in the Finder.

iPhoto

iPhoto does not have a built-in way to change the size of a photo in its library; however, it does have some options for resizing when exporting and sharing photos. First, choose Export from the File menu, or press Shift-Command-E, and then in the File Export tab, select “Custom” from the Size menu. In the field that shows up, you can set the maximum value of either the height, width, or any of these dimensions for the exported image.

In addition to file export options, you can use iPhoto’s Sharing features to export a selection of images to Mail. When you do this, you will have the option to choose Small (320×240), Medium (640×480), and Large (1280×960) as resizing options, along with an option to export them at full-size.

Mail

When you attach an image to a message in Mail, you will have options to set the attachment size to Large, Medium, and Small, to similarly reflect the sizes mentioned above. By e-mailing a photo to yourself in this manner, you can quickly set its size. Unfortunately there is no custom image sizing options here, so you are stuck with these three options besides the original size.

As a side note, if you use an iPhone or iPad, then keep in mind that you can also use these options to mail an image to someone (including yourself), and set its size accordingly.