Virtually any photo manager lets you perform mundane tasks like adjusting contrast, adding a watermark, and applying effects to your photos. But even powerful applications like digiKam and F-Spot can't really help you when you need to perform the same action (or a sequence of actions) on dozens or hundreds of photos. For that you need a batch processing utility like Phatch. This nifty tool can perform no fewer than 35 different actions on your photos, and its user-friendly graphical interface makes it easy to create advanced multistep batch rules.
If you are using Ubuntu, installing Phatch is as easy as downloading its .deb package and double-clicking on it. A source tarball is also available. If you want to be able to view EXIF and IPTC data, you should also install the python-pyexiv2 package, and for "cool Nautilus integration" Phatch's Web site recommends installing the python-nautilus package.
Unlike conventional image editing applications, Phatch doesn't allow you to edit photos directly -- instead, you use it to set up actions. An action in Phatch is a single operation that the application performs on the photos that are fed into it. Each action offers a number of options: for example, the Scale action allows you to specify the width and height, resolution, and resampling algorithm. You can add as many actions as you like. The project's wiki provides a list of all the actions supported by the Phatch.