ImageMagick's convert utility is a great way to process images on *nix machines. You can handle images in batches to, say, resize and convert an entire folder down to web-friendly jpgs, or do some more complex things like add drop shadows to images, combine multiple images into one, and so forth and so on.
The configuring and installing process for ImageMagick is fairly simple and well-documented. However, you can't just configure, make, install your way into drop shadow-ing images on the fly. The main thing you have to be aware of is that ImageMagick, unlike some other multi-media utilities like ffmpeg, doesn't include all the necessary libraries you need to do what you want.