If you have access to a lot of personal photos that are relevant to your talk (or know of photos from others that you can directly ask permission to use), we highly recommend using them for your slides.
If you don’t, or if you don’t have enough to cover all of the slides you want, you can search for free-to-use images on websites like Unsplash or Pixabay (see full list here). You can also reach out the owners of any normal copyrighted photos to ask for permission to use.
Just make sure that, with any photos you use in your presentation, you do in fact have permission to use them.