On the first day of my career at McKinsey we were told to put the sources of our analysis really prominently at the bottom of each chart. Even if the source of the data was yourself, simply put "Team analysis".
Still, many presentation slides have very conspicous sources and foot notes at the bottom. It is a typographical eye sore. When you are standing up to present your slides, people are not interested in reading the small print.
I am not advocating to take the foot notes of all together. Readers who go through the deck afterwards might be interested in them. It is also very hard to keep a good book keeping of data sources, better to have them handy all the time (I do not have the discipline to keep on updating that page with all the sources in the back of the document). And finally, taking of the reference to a photographer in an image with a creative commons license is not good practice.
So instead, do the obvious. Make the font really small (you can overwrite the "8" as smallest font size in the PowerPoint drop-down menu and make it a 6). And give the font a color with low contrast with the background. In that way, you get the best of both worlds.
This might also be the way to handle your lawyer who insists to put “confidential” and other disclaimers on every slide of the deck. Page numbers can be treated the same way. Sometimes they need to be there, but only for people who stand with their nose against the screen.