Where can you get images to decorate your blog posts? Can any of them be used for book covers, banners or business cards? Here are some ideas. I'll go beyond images and include some other media types.

General Images
If you want to re-use things then Creative Commons is one option. The Creative Commons search is really good, and lets you search different types of media (you can do the same for Flickr).

These sites are the main ones I browse when I am looking for free images or inspiration.
There are also sites where you pay for images. I have reservations about some of these.
Never forget the option of just taking a photo yourself! It is sometimes the easiest way, if you need have a good camera and want something very particular.
Other Media
Image Editing
I favour Gimp for editing images, including book covers. Gimp is a free, community-maintained tool. There are also free online options such as Pixlr, and paid options such as BeFunky - though both of those require crappy Flash. Many people also praise Canva for having good control of image and layout, Facebook/Twitter covers etc.

Always Check Permissions!
Many of the sites I have linked to offer images that are free as in "no money" and also free as in "no restrictions" (or fewer restrictions) on use. Although the quality of images may not be as consistently high as those on professional stock sites, I have often been impressed with the free ones available.

Note that not every image on the sites is available for every use though. Here we get into the issue of "licences" - the rules governing what the photographer says you can do with their image. Always look at the small print and check in case the images are free for personal use (e.g. decorating a blog post), but not for commercial uses (such as a book cover). Most sites will make the licensing information and permissions clear from a link. Do read this, and abide by it - it's not worth breaking the law for the sake of a minute checking what you can do with an image! If in doubt, go elsewhere. And be aware that even on the same site, different images might have different restrictions - some sites let the photographer decide on the particular licence. I usually make a screenshot of the licence and save it in a subfolder along with the image URL, for future protection.

Sites may well have their own licences, but many use Creative Commons, which has a variety of licences with different meanings. There's also public domain and royalty-free works. Even Creative Commons includes a Public Domain licence where the creator surrenders all rights in the work, so you can use it for any (legal) purpose. If you work in education then Xpert can be a useful tool - as well as finding images, it can build in the source, attribution and licence, so also teaches students about these issues.

Any graphics sites you use which other people should know about?