Step 3: Check your grid images!
Your grid images should now all be cropped to the same dimensions, regardless of their size when you uploaded them.
If you want different dimensions, just change the aspect ratio in the custom code.
For example, you could change the aspect ratio to 1/1 (instead of 19/12). That would make your images square.
Or an aspect ratio of 3/4 would make it a portrait rectangle.