In that case, saying you don't understand the responses you've received is a
good idea rather than just repeating your question -- otherwise you'll
continue to get the same responses that you don't understand.

DPI doesn't really matter. If you take a 1-inch by 1-inch image and set it
to 300 DPI (or PPI -- pixels per inch), you have 300 pixels  X 300 pixels to
work with. If you take that 1-inch by 1-inch image and set it to 96 DPI,
then you have only 96 pixels X 96 pixels to work with. You need an inch
measurement to go along with DPI or PPI in order to make it meaningful.

We don't care about the DPI with PowerPoint -- we care about how many pixels
you have to work with. A projected screen is generally 1024 x 768 pixels.
Forget the inches. So if you have 768 pixels on the height side, then you
have enough to size the image to the height of the screen (which is the same
as the height of the slide) without it becoming blurry.

How big is your image in inches? If you tell us that, we can tell you what
DPI setting to use to get enough pixels. Otherwise, we'll tell you to make
sure the image has 768 pixels (or thereabouts) on the height side.

You might also want to read this:

What's all this about DPI and INCHES and RESOLUTION?
http://www.pptfaq.com/FAQ00075.htm