Keeping high resolution photos is fine if you have plenty of hard disk space, but if you are going to send a bunch of them by email, you will want to shrink them first. Likewise, when you include them in a presentation, it will slow down your work and result in a huge file.
In the past it was often necessary to use a specialist image manipulation program first, to save smaller copies, before working with them in your application program. Nowadays, however, various programs can shrink the files in more convenient ways.
Image and photo programs
Specialist programs like Graphic Converter or Pixelmator are often included in special-offer software bundles. These can do what is needed and a great deal more.
However, you can also simply open each image in Apple's wonderful Preview program, then choose File > Save As or File > Export, and choose a smaller resolution. In Preview it is also easy to cut out parts of the image (click here for an example), and to annotate the image e.g. with additional text.
Emailing from iPhoto
iPhoto is another easy program that you could use. Select one or more hi-res images in iPhoto, then Share > Email, and choose from the simple list of small, medium or large size. This creates an email with lower resolution images attached.
A quick tip is to use the steps above even when you don't want to send the photos by email. You can then just drag the images out of the draft email into whatever other program needs them.
In Keynote, to reduce the file size of an individual image that you’ve masked or resized, select the image and choose Format > Image > Reduce Image File Size. (You must save the Keynote document first.)
To reduce the sizes of all masked and resized (shrunk) images, and shortened videos, choose File > Reduce File Size.
In PowerPoint 2004 and 2008, there is an option in Preferences to compress graphic files. This pdf from UC Davis, California, explains where to find it. This affects all your photos in all your presentations.
PowerPoint 2011 is more flexible (like Keynote) as it helps you shrink individual images, or all images in a particular presentation. You can click on a graphic/picture, and one option in the toolbar will be "compress". Alternatively, the File > Reduce File Size option will resize all photos within the presentation. It lets you choose the picture quality and you can crop the non-visible portions of any images.
Automator, which is included in OS X, can be used to rename and resize a batch of images. This article at The Unofficial Apple Weblog teaches you how to build a workflow for this.
Contributed by several people in discussions on the Mac Ministry List in February 2011 and December 2012.