Presenting other people's material without their permission at a conference or public lecture could potentially breach their copyright - the legal exceptions and licences which allow you to use third party material for teaching are unlikely to apply.
Using photos, graphics or audio/video recordings to illustrate your talk may be difficult to defend as 'Quotation', unless you are intending to critically review that material, and there's no impact on the market for the original. If you are using material released under a Creative Commons licence, ensure that you stick to the terms.
Be especially careful if your audience has paid for tickets, or your presentation will be recorded for wider distribution.
Read more about how to Protect your copyright before accepting an invitation to speak at a public event.
If you are the conference organizer, you should ask your speakers' permission before recording the event. Consider also asking speakers to confirm that they have permission to use any third-party material that features in their presentation.