Placing oversized images into a presentation only makes the presentation file oversized. I could suggest everyone go through my process; open each image in PhotoShop, crop, resize, optimize color, save as a .jpg or .png (as appropriate). But that is a lot of effort, and not the fastest process for a general use presentation.
Or, thanks to my brother-in-law who introduced me to Microsoft PowerToys, you could use Image Resizer. Once installed, simply right-click the image file in Windows Explorer, choose Resize Pictures and from the simple dialog choose a size and you are done! This free Microsfot application is somewhat limited in what it can accomplish, but it makes the process nearly instantaneous – produces good quality images – and works with .jpg and transparent .png images. Check out the details here.
Troy @ TLC