The lower the resolution, the worse a gradient will look - it'll look chunky and posterized if it's too low-res.

What's the res of your monitor?

I've never done a PowerPoint presentation, but think about it - your monitor cannot display anytjhing larger than it's max resolution. So the "best" resolution would be what your monitor displays (1024x768, 1280x960, 1280x1024, etc.).

If you're thinking you need to have gigantic images, don't - your graphics card will have to re-size them smaller, and you will end up with unnatural jaggies/stepping which will look bad anyway.

And actually, size them to the specific window size you will be using in your presentation. Example: - your monitor can display 1600x900, but the window you will be using is only 1280x720 kind of thing. make al your images no larger than either 1280 wide, or 720 tall - get it?

Hope that helps - good luck-
-Lew ;-)