Ideas are not protected by copyright, because copyright only protects the concrete expression of an idea. For instance, the idea to write a guide to a television series is not copyrightable, but the words used in the guide itself would be copyrightable. Of course, copyright is not the only form of legal protection. An idea might be patentable or lead to the creation of a trademark, but such cases are less likely and certainly require expert legal assistance. Often a creator may be nervous to disclose an idea for an image, a format, a product, or other creations without risking that the idea may be stolen.
Often times if you are dealing with a company that has a good business reputation having an NDA in place may be unnecessary. However, if you feel nervous about the possibility of the idea being stolen or concerned that idea is especially vulnerable then you may want to get an NDA in place.
Sometimes the opposing party may refuse to sign an NDA. At this point, the creator has to evaluate the risk. Does the company have a good reputation? Are there other companies that could be approached with the idea and willing to sign a nondisclosure agreement?