Let me repeat the definition of plagiarism as "the representation of other people's words and/or ideas as your own". In particular, in means that it is not just a technical issue (how to cite), it is equally a matter of impression that your work makes on your readers or listeners. So if they see a slide and believe that it is your own text, well, it's plagiarism.
For example, I used the definition of plagiarism above without citing it source, but it should be clear from the context that this is not my personal definition, so I think I am not plagiarising right here :)
Personally I even think you can simply acknowledge the source verbally, but in this case you have to be sure that the presentation will never be used apart from your talk (which is hard), so I wouldn't recommend it. I agree that in case of presentation you probably won't get into trouble, but if I see one "borrowed" slide without acknowledgement, I might presume that the whole presentation is "borrowed" from different sources, so it's not in your interest.
The particular way of citing is not important: it might be just a question of your page layout. If you don't have enough space, make a footnote "taken from " and expand it in the last slide. If you have space, provide a URL right on the slide — it's fine, too.