Public domain image library are free to download for personal and commercial use. It's hard to search good images that are 100% free and most of the time backlinks are not required in using these images.
1. 4FreePhotos – This has few artistic public domain image good for background because the images are in good resolution. The group of photographers decided to share their shots for public domain and added it to their library.
2. Karen Whimsey Public Domain Images – The photos are composed mostly of clipart, diagrams, sketches and more for the classroom. It is a library of image for ancient civilization, musical instruments, world religions, silhouettes, vintage fashion, art, fairy tales, Native Americans, Civil War, Geography, food, alphabet, plants, holidays, etc.
3. Photos8 – The images here does not require a backlink. The images are in high quality and resolution which is also ideal for desktop backgrounds. You can search the image library in different categories from this website. Some image seems not to be a public domain photo because it is really in great quality.
4. Pics4Learning – This is a public domain for learners and teachers. It is copyright-friendly which mean that it can be use for commercial or personal. All images are donated by amateur photographers, students, and teachers. Not all images here are in high quality.
5. Public Domain Photos – This public domain image library consist of 5000 photos and 8000 cliparts to search and can be use for any purpose but be careful with some photos that contains branding like logo and trademarks when using it for commercial use.
6. Uncle Sam's Photos – This is a library of US Government's photos, it is a image library where you can search public domain photos.
7. Backgrounds Archive – This is a nice library of background images. You can download free tileable, desktop wallpapers, and Myspace backgrounds for wide-screen and regular resolutions.
8. Morgue Files – The images here are good for inspiration, reference, and for creative work. It can be use in any purposes. The photos are in good condition but not all are in high quality. You can search the image in library using advance search to filter your keywords.
9. Icon Finder – Here are large collection of icons that can be sort by license. You can search icons that are free for commercial use and don't need any attribution.
That's it. I hope that the links above can help you a lot specially when you are searching for free to use images for your website or graphic works. Feel free to bookmark this page and share this to your friends instead of keeping doing a search for images that can be use for free. I find this list very helpful when I am designing websites.