A collage is an combination of multiple images arranged in a single image. Collages are often considered derivative works of the images contained within, which can however often cause licensing conflicts.
License compliance and collages 
To comply with the licensing of the images used in a collage, two things will often be 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 a 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 the exact same license (or are in the public domain), its easy as long as you attribute them. While the recent GFDL/CC-BY-SA licensing transition has made this process a bit less daunting, there is one important rule to remember: Viral copyleft licenses cannot usually be combined together. For instance, you cannot take a image only licensed under the GFDL and combine it with a CC-BY-SA only image, since the viral copyleft requirements conflict with eachother. 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. But even with that, you cannot include a image licensed under the GPL in the same image since it is another conflicting viral copyleft.
Combining different Creative Commons versions 
- Creative Commons Attribution (cc-by) regardless of version can be combined with any other creative commons license/version the resulting license having the version number belonging to the most restrictive license.
- 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-2.0
- cc-by-2.0 + cc-by-sa-1.0 = cc-by-sa-1.0
- Since version 2.0 and greater, all the Creative Commons Share-Alike (cc-by-sa-2.0+) licenses have a clauses that allows derivative works to be placed under the same or newer license. Any combination of versions the resulting license will have largest version number.
- cc-by-sa-2.0 + cc-by-sa-3.0 = cc-by-sa-3.0
- Creative Commons Share-Alike version earlier than 2 can't 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 it under the CC-BY-SA license. It's the better option in any case so...
Public domain images like the one below have no license for things to conflict with 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
However, you cannot include this fun star icon in the resulting image because it's licensed under the GNU General Public License (but then again, why you'd need to have that specific image in there, I don't wanna know). You could only "put" it on a public domain image! CC-BY and other "attribution-only" images are a grey area, they don't have a copyleft component, so it is believed that they may be combined with any license as long it is attributed properly.
For obvious reasons, collages on Wikimedia Commons can never include non-free images, they are simply not allowed on Commons in any form, and making a collage with one in it will not change its licensing status, and will make the entire image non-free.