Human beings are visual creatures. We love a photo (especially of ourselves, but let's move on!) and images in your promotional items can really add to the professionalism and appeal of your materials.
These days it doesn't even need to hurt your budget to use lots of pics in your web, email and print comms due to a fab site called www.istockphoto.com. If you have yet to visit this wonder, they have over a million royalty free images, which you can buy in low res for US$1 - I don't really need to say much else, as it doesn't get much more zero budget than that!