Conference presenters must adhere to the copyright laws of the conference host country. Best practice is to consult the conference organisers about what copyright obligations may be in that country.
In many cases, the conference proceedings may be published, either openly online or in a subscription database. If your paper is going to be published, then your need to ensure that any third-party copyright material (not written by you personally) is done so with the relevant permissions.
International copyright agreements and treaties provide some basic guidance and common principles upon which to act. One important treaty is the Berne Convention which provides common standards of copyright protection for member countries. 180 countries have ratified this treaty, which is administered by the World Intellectual Property Organization (WIPO).
Useful websites:
Learn more: RightsDirect: International Copyright Basics