It is tricky in PowerPoint to make a nice grid of images that comes from different sources, in different sizes, and in different aspect ratios. How do you get them all the same size? It can be very tedious to crop them all to the same proportion, and then line them up correctly. There are always one or two that are wrong.
Here is what I do. Crop each image to a certain aspect ratio, don't worry yet about the exact size. Now select them all and give each the same height, the width will automatically be adjusted as well! Pro-tip, crop to 1:1 and then try cropping to a circle.
In my presentation app SlideMagic, it is impossible not to lay out images in a grid :-)