Making a grid of images in PowerPoint is tricky. Images never have a consistent aspect ratio, and when you place a lot of them on a page, the guide suggestions always snap in the wrong place somehow. Here is a survival guide.
- Copy all your images inside the page and select them all
- Right click and go in "format picture"
- Tick the "size" icon, and click "size"
- Hit "reset" to kill any aspect ratio distortion
- Hit "lock aspect ratio"
- Now select each image one by one, hit "crop", hit "aspect ratio" and pick one
- After this, select all the images again, and give them the same width with a numerical value
- Position the images on your grid
- Take each image in turn, select "crop" and move/zoom the image mask for the right composition
The above was a major consideration when designing the image grid system in my presentation app SlideMagic.