Legally, you are responsible for ensuring YOUR work follows copyright. If your work is made of other works then you need to ensure you have a license for them all. Otherwise you can be sued and saying someone uploaded it to Flickr with a CC-BY won't save you. Presumably you'd track down the original author and have them sign a piece of paper where they claim it is their work. Then you can at least sue them for damages if it's not.

edit: As for your example, copyright generally doesn't care about you consuming a work but about you sharing a work. As such, you don't have to check anything but Disney does have to check everything.