A PowerPoint or other visuals should be used if it helps with the:
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interest,
engagement,
clarity,
organization,
of your speech.
For sources on a PowerPoint, you should state "Source: __" or "Photo Credit:. ____."
You can also use one of the standard citation formats (APA, MLA, Chicago, etc.) Often,
you will create a hyperlink (not the URL) behind that source; though, it's not always
required.
It gets trickier when your source is using someone else's material. When possible, cite
the owner of the original material/photo. When not possible, cite the original creator of
the material/photo and then add something like, "via YouTube Payne's Anime
Channel."
If you are using a stock photo from a free (or paid) stock photo site, you are not
technically required to cite it; however, I recommend doing it to prevent me from asking
what your source is.
You should also cite yourself (and the date taken/produced) if you are the photographer
or the creator. It's safer to cite than to not cite.
Now, it gets even trickier if you are using the content for commercial or business
content. You may not use the photo/graphic for financial gain or commercial
purposes without getting licensing rights or written permission from whoever owns the
photo/content. However, Fair Use laws allow you (in most cases) to use them in
educational setting like ours, without permission.