I've been in many situations where the copyright owners reached out for damage fees after downloading a full-res, un-watermarked photo from free stock photos sites in blog posts, so I'm sure the tech is all there already.
These are all situations where the copyright of a stock image owner is infringed and yet there is very little that automated processes can do to detect them.
1) No one outside of the 5 people attending the presentation will ever know. The 5 colleagues following the presentation absolutely don't care where the images are coming from, as long as the point is clear and I'm speaking loud enough.
2) Redistributing ? Really ? If I'm sending a cat picture to my mom, my manager or my favorite slack channel, I'm "redistributing" ? Come on. It's not a publicly available blog post, it's my inner social circle.
I'm never going to pay $30 for the few images I used to make my presentation less boring. However, I can make a little effort and put a 12pt "credits" slide at the end (usually, no one care about).
a. Copyright isn't about audience, it's about author. Ask if the person whose work your using would care, not the people seeing it
b. copying is copying every time, not just after the n-th time
c. credit acknowledgement is good, but it doesn't pay the rent for people who work to create content
d. a rough rule of thumb would be whether you're using art to support a profit motive. Anything that happens for your work would be considered in support of a profit motive.
e. "ask" in point a is not used in the figurative sense
Additionally, DMCA is only valid within the United States.