Correct resolution?

Question

I'm still a little stuck on preparing graphics for use in Keynote. I had a bad experience with using images that were way too big, and the presentation was slow to load resulting in a very squirmy audience. Not good.
So. I have a huge aerial photograph that I need to insert into Keynote. It's about 6000x6000 pixels. Big.
Do I just keep reducing the size in Photoshop until it fits into a Keynote slide?
I have a sense that there is an underlying principle here that I am not grokking.
MIchael

Answer 1

Michael Spencer wrote:
I think I had trouble visualizing. I figured that if I put a small res image in Keynote that when it is projected and therefore enlarged to a huge size on the screen, it would pixilate or blur or somehow else have poor quality.
As long as the resolution of the image is the same resolution as your slide, it will look as good when projected as the rest of the slide. There's really nothing fancy or magical going on -- if your slide is set to 800 X 600 (or 1024 X 768, or whatever), then that's the maximum size your image should be. Any bigger and you're just throwing pixels away (and relying on Keynote's resizing algorithms) while increasing file size.
Now, for maximal quality, you ideally should size your slides to the "native" resolution of the projector you'll be using. Just like LCD screens, which have a certain number of physical pixels horizontally and vertically, and must interpolate if you run the display at a different resolution, so to with projectors. Most newer projectors these days have a "native" resolution of 1024 X 768 (older ones are 800 X 600), and if your Keynote presentation uses slides of a different size, the projector will re-size them, reducing the quality somewhat.
But in the end, projectors are only going to display as many pixels as they can natively show on a screen, no matter how large or small that screen is. So, if you've got a 1024 X768 projector, and your slide is set for 1024 X 768, and the image filling your slide is at 1024 X 768, that's the best you're going to get in terms of resolution.