If you need to find high-quality images to use on your website, but you can’t (or won’t) pay for them, the best solution is to get images from websites that offer free (for commercial use) images.
There are many websites that do that, but, in many cases, these websites have low-quality images and/or low resolution. Or, what is the most important, the license some websites use is not clear enough, and images are not really free to use in commercial products. What we really want, are images licensed under CC0 (Creative Commons Zero) license, or with explicit permission to use them for free, in commercial projects without attribution or images licensed as Public Domain.
Most websites with free images, use CC BY-SA 2.0 license, that requires attribution, and it is not compatible with GPL license. CC0 is the only Creative Commons license compatible with GPL license. So, here are 7 websites with high-quality images, all under Public Domain or CC0 license, and free for commercial use, with no attribution required.
Use comments to let me know if you have some other CC0 photography websites to recommend.