If you ever need to invert the colors in an image, you probably do so in your usual image editing application. If that program is Photoshop, and it’s running, no problem…but if it’s not running, you’re in for a bit of a wait while the program loads.
As an alternative, at least if you’re running OS X 10.5 or later, consider using Preview. Open the image in Preview, then select Tools -> Adjust Color to open the floating color adjustment panel. On that panel, notice the sliders for Black Level and White Level.
You may also find the results more pleasing if you lower the Brightness setting from its default level.
Preview won’t ever replace a high-end photo editing application, but in its latest incarnation, it’s a surprisingly powerful program. Thanks to Mac OS X Hints reader Lukas Machata for pointing out this somewhat-hidden feature.