its are 4 rows because your monitor is so wide if you resize the codepen it will show 1 or 2.
on iphone it will show one picture
on ipad i’m not sure(i don’t remember size of ipad :-D)
i took your css so it should be centred. only problem if the monitor is too wide it will display more.
the problem was your collection-repeat implementation:
i change it to this:
collection-repeat="post in posts"
with my working % do not work well with collection-repeat, he likes more fixed with.