As part of a survey (collecting environmental data from the public) my participants can choose to provide their name or to contribute anonymously. Part of the survey includes submission of photos that we wish to be able to reuse under the terms of a CC-BY license.
To comply with that license we need to acknowledge the source of the photos wherever they are re-used - which is fine for named contributors, but potentially problematic for users who wish to remain anonymous.
This must be a problem that many others have encountered, what is the simplest solution that respects the licence but avoids too much special-case legalise?
Can an anonymous CC-BY work be legitimately 'acknowledged' using something like "anonymous my_survey participant"?
If not, is there a simple way to allow CC0 to be applied to anonymous photos while using CC-BY for the rest? Excessive legalise will scare away many of my users.