I am working on a Beamer presentation for less-technically-inclined audience, so I want to use many visual aids, pictures, stock photos, etc. Unfortunately, these are mostly raster images, not vector diagrams.
From your experience, what minimal resolution is required so that an image would not appear pixelated when covering a full slide? I particularly refer to projecting the presentation from a beamer, not viewing it on a computer screen.
Best Answer
I do beamer presentations with lots of art. While I have not experimented to find minimum values, I get good results from anything 1024x768 pixels and up. This is flickr's "large" size.
I mention flickr because it's a good resource for this kind of thing. Do an advanced search and check the box for Creative Commons-licensed photos.
In case you want to keep looking for vector art, try these:
Free Vector Resources For Designers in 2011 Showed up in my tweetstream the other day; lists many sites. I don't know what the licensing is for these.
openclipart.org has only public-domain works. You can even search this site within InkScape to make your own modifications.