Step 1 - Fix image ratios without messing up their sizes
Here's the trick. Right click on an image and select Size and Position. From this dialog, click in the Height box. Now, just click up once and down once. As long as the Lock Aspect Ratio checkbox is checked, just changing the scale by 1 step and then switching back will fix your image. You can do it with your keyboard even faster.
Here's trick #2. You don't need to close the Size and Position dialog. It's modeless. You can leave it open and go from slide to slide quickly changing your images. Just click the image, click Height, then up/down, then do another image. Repeat until it's all done. It'll take just a few seconds per slide.
This trick will fix all your image ratios, but expect to do one more pass to make sure that you're using all the space afforded by this new widescreen layout. Regardless of how you images are sized you might want to make some tweaks to the layout and spacing of your text and images.
Sidd
"Life moves pretty fast. If you don't stop and look around once in a while, you could miss it." ..... Ferris Bueller