A collage is an combination of multiple images arranged in a single image. Collages are considered derivative works of the images contained within, so they must comply with any binding attribution or copyleft requirements.
License compliance and collages
To comply with the licensing of the images used in a collage, two things are required:
- Attribution of the source images
- Correct and compatible licensing
The first one is a no-brainer, simply providing links to the images used will be enough to comply. This is often required for attribution licenses, and is also required in order to verify if the images are freely licensed in compliance with Commons policy. However, the second one, is a little more tricker, as it will often involve dealing with conflicting copyleft licensing terms.
Although all images on Commons are required to be freely licensed under free content licenses, this does not mean you cannot simply combine an image with another image. License compatibility will often affect what images you may use together. If all of the images in your collage are licensed under exactly the same license (or are in the public domain), it is allowable as long as you attribute the images used. Viral copyleft licenses (such as the GNU General Public License, and images licensed only under the GNU Free Documentation License) cannot usually be combined together. For instance, you cannot take an image only licensed under the GFDL and combine it with a CC-BY-SA only image, since the viral copyleft requirements conflict with each other. In order to be legal, at least one of the images must be licensed under both licenses, so the resulting collage is only licensed under the common license. CC-BY and other "attribution-only" licenses do not have a copyleft component, so it is believed that they may be combined with any images of any license as long as they are attributed properly.
Collages may not incorporate non-free images. Making a collage that incorporates non-free images does not change their licensing status, as they are an unauthorized derivative work. Hence, if a collage contains a non-free image, the entire image becomes non-free and cannot be hosted by Commons.
Combining different Creative Commons versions
A compilation can be created between images using any version of the Creative Commons Attribution (CC-BY) and Attribution Share-Alike (CC-BY-SA) licenses, but the final license for the compilation itself has to be the most restrictive of the licenses used;
- cc-by-1.0 + cc-by-sa-2.0 = cc-by-sa-2.0
- cc-by-sa-2.0 + cc-by-3.0 = cc-by-sa-3.0
- cc-by-2.0 + cc-by-sa-1.0 = cc-by-sa-1.0
Images licensed under CC-BY-SA 2.0 or newer can be incorporated into derivative works that use a newer version; in these cases, the final license of the compilation is the most recent version of the license used among the images
- cc-by-sa-2.0 + cc-by-sa-3.0 = cc-by-sa-3.0
However, Attribution Share-Alike versions earlier than 2 cannot be combined with version 2 or newer.
- cc-by-sa-2.0 + cc-by-sa-1.0 = incompatible.
Say we wanted to make a montage of images of Toronto. Thankfully the following three images are licensed under both the GFDL and CC-BY-SA licenses, so you can legally use them together.
These images are only licensed under the CC-BY-SA license. IF you were to combine them with the above images, you could only license the compilation under the CC-BY-SA license (however, as the GFDL has been effectively deprecated, this is actually a preferable outcome).
Public domain images have no attribution or copyleft requirements at all, so they can be incorporated in any collage. However, you cannot distribute the collage as a public domain work unless all of the works in it are in the public domain.
You cannot include this fun star icon in the collage because it is licensed under the GNU General Public License (but then again, why one would need to have that specific image in there is anyone's guess). Even though it is an also copyleft license, the GNU GPL contains additional provisions that make it incompatible with the CC-BY-SA license.