A caveat right off the bat: If you want great photography that perfectly illustrates your website or publication, you should expect to pay for it. I'm not trying to run photographers out of business -- it's a profession that doesn't get the respect it deserves, IMHO.

But that said, sometimes you don't have something specific to illustrate. Or you don't have a budget. Or you need something random/generic that likely already exists.

So what do you do?

In most cases, if you're a commercial enterprise, you purchase some stock art. Stock art is a lot less expensive than setting up a photo shoot, and it suits well for many purposes.

What if you LITERALLY have no budget?

Well, that's when you have to get creative. Despite a still-common belief to the contrary, images you see online are NOT free for the taking. Unless you're notified otherwise, you should assume that any image you see on a website is owned by someone. And they'll either have to grant you permission to use it for free, or they can charge you to do so.

Here are three choices to consider for finding great free images:

1. Take the photo yourself. This is simpler than ever, with point-and-shoot digital cameras creating great images at a low price. If you have a digital SLR and know how to use it, even better. Follow these tips for a great photo.
2. Check Flickr. Whatever you want a picture of, rest assured, it's already on Flickr. The pages for individual images on Flickr will display the rights available for the image, so that's helpful. Even more helpful: Search images available for use, if you attribute the photo, from this URL: http://www.flickr.com/creativecommons/by-2.0/. Be sure you're properly attributing any image you find on Flickr, of course!
3. Try Stock Exchange. Created specifically for photographers to share their work for free, Stock Exchange is another great resource. As always, make sure you're meeting the requirements of the photographer before using a shot.