if you are not distributing, selling, or making money with the presentation I would say putting the source where you got it from in the presentation is enough for pretty much any online Go material you can find. pictures, problems, video you can freely access on the internet (youtube vids are fine, but I would for example not use material from
Guo Juans internet Go school that are available only to those who pay for it. ect)
sensei's is a nice source.
wikipedia also has a nice article with general info on the game.
an image search on google will probably deliver some nice (ancient) Go pictures.
edit: If you do plan on distributing, selling or making money with it there is still a lot of free material out there that is publicly available (senseis and wiki, ect) if you have doubts just contact the author/owner. I did so for some of the material that is on the CD I sell with my book. In general people are only to happy to find out that their material is going to be used.