In Photoshop, open the image.  Next go to IMAGE > IMAGE SIZEas seen below:

In this menu you'll see several features, all which pertain to image size and quality.

Turn resample image off. The key is to take high quality images to start, and then reduce or enlarge.  You'll notice when changing values in the document size (width, height, resolution) that all the other values change proportionally. 

For example, if you have a 6" x 6" image that was taken at 150 dpi, without resampling, and you enlarge to 12" x 12" your dpi would reduce in half (75 dpi).  For best printed results keep the dpi around 300 dpi, but you can "get away" with 150 dpi.  This is entirely up to you, it depends on how critical image quality is.

• On screen dpi = 72 dpi
• On Print dpi = 150-300 dpi

When you add images to PowerPoint, do not just copy and paste them in.  When you copy and paste your images, often times you're getting a 72 dpi copy of the high quality images you wish to place in your document.  However, if you are using excel charts/graphs/etc., they are vector based and can be copied and pasted into the PowerPoint document without a problem.