Image grids are fixed grids where you can place images. They are great for creating mood boards.
You can change the color and thickness of the frame here as well. The image grid offers an easy way to change the look and feel of your design.
Moving images within the image grid is easy - just click on the image and move it:
One thing to add, if you create a design or pick a template first and then add image grids, the grids sit behind the designs, so it is best to choose either an image grid or a template first.