If your non-profit has a blog, you may need imagery periodically to illustrate a post. Here are a few resources to help you locate free (or low cost) imagery. (They can potentially also help with other projects, but make sure that you don't use an image just because it is free when you can find another image that would make a campaign or advertisement much more effective).

Free images: public domain, creative commons and other

You can find many images that are freely available for use. The quality of the free images is often not as good, especially at full resolution, but for some uses (such as illustrating a blog with a low res image) they can be just fine. Here are a few sources for imagery that is often public domain or made available with a creative commons license or another free-to-use license. Please add more suggestions in the comments and I'll update the list periodically.

1. Sprixi is a new image search engine, specifically with free imagery in mind. It claims that it gets better after each search because it invites users to help rank images which are useful or not useful for a particular term. You can also download the images in various sizes, with or without an appropriate credit already added at the bottom of the image. ("The images are sourced using the Flickr API and various other places but all content has a liberal licence such as Creative Commons or is in the public domain. From the looks of things image tags are used as the primary filtering tool.  On top of that, as the site FAQ explains, images are sorted automatically just by people using Sprixi. Images you click on, rate, use, download or upload are given a usefulness rating. You can also vote for the usefulness of images by clicking on the yes, maybe and no buttons. Votes from registered users are given a heavier weighting." -thenextweb.com)
2. You can always go to Flickr and do some searching, although you'll need to keep an eye on the licenses that the images are listed under. If you find just the right image with the wrong license, you can also contact the user to see if they will grant you permission to use it -- being a non-profit can often help in getting permissions like this.
3. Stock.xchng is a free image site that was bought out by iStockphoto. It is currently still free to use, although like Flickr, you need to keep track of the usage license that is associated with each image, and be aware iStockphoto also feeds images from the for-pay microstock images when you search. (Those images are still very affordable -- see the microstock section below).
4. Dreamstime is another microstock agency that has a free image area. They will also have a two-tier search result -- one with the free images that match the search and the other with the for-pay images.
5. Many of the images that are used on Wikipedia have free use licenses. To try to find these types of images, you can do a Google image search and specify that you want to see results from Wikipedia by using a special filter such as "site:commons.wikimedia.org" before your search query. (Side note: you can also sometimes find SVG, or scalable vector graphic, files of things like country flags, which can be very useful).

Low cost images: microstock

The microstock photography model has exploded in recent years and it can be very useful for a non-profit (especially when money is tight). The quality of the images is getting better and better and the cost is still very affordable, especially compared to traditional stock photography. If you are planning to use an image in a blog post you can often get away with the lowest resolution they offer, which may cost in the neighborhood of one dollar on many of the websites. A few of these sites are already mentioned above since they have free image sections.

Learn more about purchasing images from microstock sites (such as Shutterstock, Dreamstime, iStockphoto, and Bigstockphoto) here.

Conclusion

You can find a lot of free-to-use imagery online, but always keep in mind the license that it is offered under. You may need to find a for-pay image (either through microstock, traditional stock or even a custom photo shoot for specific projects). A recent study in the UK showed that over one third of creatives in the UK use images illegally, and even major companies get caught doing so. Don't let your non-profit get pie-in-the-face by using an image you don't have the rights to use -- it's not worth it and will damage your credibility