If you blog, you probably know that most online images are copyrighted and off-limits for your site. Where is an enterprising science writer to turn for artwork that is free, beautiful, and legally bloggable?
1. Ask the artist
Artists own their copyrights, but that doesn't mean many aren't happy to share! Often, permission for non-commercial or personal blog use costs a mere link back to the artist's website. While taking 30 seconds to compose a brief email may seem like extra work, consider that symbioses between writers and artists often benefit both (exhibit A: Primate Diaries blog and Nathaniel Gold). Don't be shy!
Government agencies place most of their images in the public domain. As they should- the public paid for them! NASA maintains a wondrous database of space-related imagery usable for most purposes so long as you do not imply government endorsement of a commercial product.
The U.S. Geological Survey curates a sizeable collection of public domain photographs covering not just rocks, volcanos, national parks, and earthquakes, but fascinating early images of native cultures, political figures, historical events, and more. Like NASA's archive, USGS photographs are free to use without prior permission.
4. NIH Images from the History of Medicine
The National Institute of Health's historical archives contain 70,000 images, including photographs, cartoons, paintings, public health posters, and other miscellanea.
5. Public Health Image Library
The Centers for Disease Control hosts PHIL, the Public Health Image Library. Most of the content is an unusual combination of being both modern and in the public domain, but check the details before use, as a few of PHIL's treasures are copyrighted.
6. Wikimedia Commons
The Wikimedia Commons is a giant repository covering quite literally millions of items of varying copyright status, including public domain and an assortment of Creative Commons-licensed copyrighted work. Creative Commons material may or may not be safe to copy depending on the particulars of your intended use and the rights-holders' interpretation of "non-commercial", so I recommend sticking to the public domain.