I have quite an old version of PowerPoint so I don't know if my comments are still valid. Gamblingman's method is quite good and parallels my method. You can improve quality fairly easily.
My PPT gives slide jpgs at 75% quality, 2:2 subformat and they are 793x595 pixels. 75% is poor. The 2:2 subformat is crappy for sharp detail like text, giving rise to "mosquitoes" and "edge halo". The png output is better quality but at the same dimensions. In my version of PPT you have no control over jpg size and quality so png is preferred.
An alternative is to use a capture utility (loads out there). I use FastStone. Set up to capture fullscreen, bmp format. Run the slideshow and capture the slides as you go. Bmp format does not suffer from degradation and you have slides at screen size. The only loss is PPT sizing to the screen. You can stick those bmps into your graphic editor. Png or bmp should both be fine.
Edit: A further advantage of screen capture is that you can duplicate certain animations. Suppose one slide has text appearing, followed by 4 images in sequence, you can capture each stage for a total of 6 captures. Saving that slide as a jpg or png with PPT produces one image with everything on it.