I will admit that I am very dismayed by this. Yes, I too sometimes do use other people's figures and plots in my talks, but I usually add a source, if possible to find. It's more complicated with photos, who will typically appear in so many copies on some dozen websites that it's next to impossible to find out who originally took the photo. In any case, some of the pictures I saw reappearing in those talks I don't even hold the copyright on. They were published in one of my papers, and with that the copyright went to the publisher.
I don't mind at all if people use my pictures, otherwise I wouldn't upload them to my website. I receive the occasional email from somebody asking if they can use one or the other for a talk or a paper and I always say yes. (I once was asked for a picture to be reprinted in a popular science book, but when the publisher of my picture was asked for the reprint permission they said no for reasons I still don't understand.) But of course I do expect that people add at least my name below it. It has previously happened that I saw pictures of mine reappear, this one showing an evaporating black hole seems to be the favorite
but that workshop convinced me to add my name in a corner of all these pictures. Sure, one can cut it out, but it takes a deliberate effort.
This also reminds me that I once received a paper for peer review. It was written in dramatically bad English, then all of a sudden there were two paragraphs that weren't only readable but sounded eerily familiar. A quick check confirmed my suspicion that it was an introduction from one of my own papers. They had cited my paper somewhere, but it was by no means clear they had copied half a page from it. Again, my paper was published, the copyright was with the publisher. The paper I reviewed wasn't only badly written but also wrong, so it didn't get published. However, I later wrote to the authors making it very clear that this is not an appropriate way to cite. They either mark it as a quotation, or they rewrite it. They apologized and then rearranged a few words here and there. I know other people who have made exactly the same experience with one of their papers.
I find it very worrisome that more and more people make so unashamedly use of other's work without even thinking about it. My mother is a high school teacher and as a standard procedure she'll have to check every essay for whether it's been copied elsewhere. Evidently, there's still kids stupid enough to try nevertheless. I know these checks are being done in many other places too, there's even software for it so you don't have to Google every sentence manually. An extreme case that I know of was a PhD candidate who had copied together half of his thesis from other people's review articles, including equations, references and footnotes. He did cite the papers he used, but certainly didn't mark the "borrowed" pieces as quotations.
It is clear that when thousands of people write introductions to the same topic, then many of them will sound quite similar. I also understand that when you find a nice picture for your talk online it seems superfluous to spend time yourself on what Google gives it to you on a silver plate. Certainly you have better things to do than making a pictures for your talk, right? But what you're doing is simply using someone else's effort and selling them as your own. So next time, spend the three seconds and check whose homepage you've been downloading your pictures from.
And here's a recent copyright story that I found hilarious "Greek man sues Swedish firm over Turkish yoghurt pic"
"A Greek man has sued a dairy firm in southern Sweden after his picture ended up on a Turkish yoghurt product. The man whose picture adorns the Turkish yoghurt product, manufactured by Lindahls dairy in Jönköping, argues that the company does not have permission to use his image [...]
The man, who lives in Greece, was made aware of the use of his picture on the popular Swedish product when an acquaintance living in Stockholm recognized his bearded friend [...]
In his writ the man has underlined that he is not Turkish, he is Greek, and lives in Greece, and the use of his picture is thus misleading both for those who know him and for buyers of the product.
Lindahls dairy has expressed surprise at the writ and argues that the image was bought from a picture agency [...]"