Normal practice for artists wishing to use copyright protected material in their own new works is to write to the copyright owner, explain what the new work will be and how it will be shown, and ask for a copyright license to make the new work.
In the case of non-profit activity, this may be granted for free. A license will usually explain under what circumstances the work may be shown, in what areas of the world and for how long.
You should always write to the copyright owner for their permission, explaining why you want to use their material in your work, how and where the work will be seen, and whether it will it generate income for you. If it’s a non-commercial project, the chances are good that permission will be given without requiring payment, but with an acknowledgement being given. Conversely, if it is a commercial project then the copyright owner is likely to want a one-off fee or income from a licence.
In the case of photographs you would usually contact the photographer, but sometimes a publisher owns the copyright. If the photograph is of a model, you should contact their agency as they are likely to have agreed limited specific use of the model’s image and you may be required to pay a fee for any additional use.
The issue of whether or not new photographic images of an older two-dimensional image can acquire copyright protection, on the basis that it is not a copy but an original work, has yet to be decided by the UK courts; however the US courts have decided the issue.
Genuinely found images are unlikely to give rise to copyright issues. However, where images have been photocopied or found on the Internet, permission should still be sought from the copyright owner.
Film copyright may be owned by the filmmaker, production company or distributor, or part-owned by any combination of these. A writer or publisher may own literary copyright.
For permission to use music, you should visit the website of the Performing Rights Society. They offer user-friendly information about how to get copyright licences for the use of music, in particular for use in retail products such as DVDs, and can tell you the likely fees involved. They also offer a licensing service.
There is also the related issue of the moral rights of the creator, who will have the right to object to derogatory treatment of his or her work, which will include its modification.