How to add a copyright watermark to an image?

Yes

iPhoto can not - I think Aperture can but I seldom use it so can not explain - there is an Aperture forum for Aperture questions - give it a shot

Preview can annotate photos - you can set it as an external editor for iPhoto and add watermarks using it

There are several third party watermark programs that will work with iPhoto

LN

It can be done in Aperture or with an app like these

Impression

iWatermark

There are others. Search on Macupdateor the App Store

My two image editing tools so far are iPhoto and Aperture.

Aperture can add watermark images, when you export from Aperture. You have to duplicate an export preset and create a custom export preset to add the watermark. In the "File > Export" dialog select "Edit" as Export Preset. This will bring up the edit dialog for Export presets:

What neither Aperture nor iPhoto can do, is create watermark images with a transparent background.

The simplest method on a mac to add transparency to an image is Preview, that comes with every mac. In Preview use the command "View > Show Markup Toolbar" to show the editing tools. Then use the "Instant Alpha" tool to make the background transparent.

Without a $$$ investment in Adobe tools

For more advanced image editing with transparency and alpha layers you could also try the Gimp http://www.gimp.org/downloads/. It is a free, really phantasmic open source program. It can do most of the things Photoshop can do, but has a similar steep learning curve as photoshop, because it is so powerful.

For all kinds of text annotations try BorderFX. It is Shareware.

http://www.iborderfx.com/BorderFX

I'm currently working with Watermark PRO which is free, but somewhat hobbled

Thanks for this recommendation. Good to know.