Yesterday, we featured WP Inject, a plugin that makes it easy to insert Creative Commons licensed images into WordPress content. High quality CC-licensed images are incredibly easy to find these days. The vast majority of the collections include images that fall under the Creative Commons Attribution-Sharealike 2.0 license. While these images are fine for blogging, they cannot be distributed with GPL-licensed software.

Why Creative Commons 2.0 Is Not Compatible With the GPL

When I submitted my first theme to the WordPress Themes Directory, I learned that most CC-licensed images are not compatible with the GPL. At first it seemed odd to me that they wouldn’t be compatible, given that they both offer so much freedom.

Samuel “Otto” Wood explained why certain Creative Commons licenses are not compatible in an answer on the WordPress.org support forums:

Because CC-BY-SA cannot be sublicensed, then that means it cannot be distributed in a work under the terms of the GPL. So it cannot be included in a GPL-compatible work-as-a-whole.

If you want to include images in a GPL-licensed work, you’ll need to select from those that fall under a compatible license.

According to the GNU licensing classifications, CC0 is the only Creative Commons license that is compatible with the GPL:

CC0 is a public domain dedication from Creative Commons. A work released under CC0 is dedicated to the public domain to the fullest extent permitted by law. If that is not possible for any reason, CC0 also provides a lax, permissive license as a fallback. Both public domain works and the lax license provided by CC0 are compatible with the GNU GPL.

If you are distributing a GPL-licensed product with images, such as a WordPress theme or plugin, your best bet is to opt for using images with the CC0 or Public Domain license.

13 High Quality Sources for CC0 and Public Domain Licensed Images

CC0 and Public Domain licensed images are not as easy to find as those that fall under the CC 2.0 license. Nevertheless, I was able to locate 13 high quality sources for finding images that are suitable for inclusion in GPL-licensed works. All of the following sites explicitly state a Public Domain or CC0 license for the images collected.

You’ll want to bookmark these sites for the next time you’re building a WordPress theme or plugin for distribution on WordPress.org. Do you have any other favorites to recommend? Please share them in the comments.