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.