Meetable events allow uploading images, both a banner for the event itself, and photos of the event afterwards. There should be a setup feature to explicitly pick and set one or more required licenses for image uploads.
At a minimum, Meetable should allow choosing a Creative Commons license like Flickr does (radio buttons), perhaps defaulting to a CC-BY-NC license like the Wikimedia upload default, to encourage compatibility with the broader Wikimedia commons, so images uploaded to default Meetable installations can also be published to Wikimedia, and to allow Wikimedia images to be used for Meetable event banners.
Maybe allow multi-licensing as well, e.g. picking more than one license (checkboxes), so uploads are required to be multi-licensed.
Additionally, consider allowing a user to enter one or more license URLs, so those setting up their own Meetable can choose other licenses beyond a predefined set of Creative Commons licenses.