To create an even grid similar to the featured image thumbnail grid, you must ensure a fixed height for all of the elements and, if displaying images with different widths, you may also choose to ensure a fixed width for the image.
To create an even grid, set a height for all of the item elements (or use items that have even heights). Use the default image height or set one explicitly. Set a height for the caption and for the excerpt if you are using them.
