how can you save keynote slides as high resolution (300 dpi)
to save individual images from each slide in Keynote; file > export > images > TIFF (for optimum quality)
You are using the wrong terminology to describe creating an image file
- dpi ( meaning dots per inch ) is the measurement used in printing to create the number of print dots per inch that will be deposited on to physical media like paper or film This is done in the printer settings box of the print driver and is dependant on how the individual printer can print..
ppi ( meaning pixels per inch ) is the measurement you need to use when creating the number of pixels in a digital image file. You establish image resolution in Keynote using the inspector;
Keynote > inspector > document > slide size
I can increase the dpi in photoshop, but it doesn't produce crisp images for obvious reasons.
You can resample images in editing applications such as Photoshop or Painter but what you are doing is adding non image information to the image to increase pixel count, this is called noise. As you state this is not the procedure to use in your situation.