Max number of content per row: the number of products is shown on a row, Eg: 5.7,2.2 corresponding with the desktop, mobile.

    Image manual height: Input a value to set the manual height of image. Eg: 150px

    Image position: Input the position of the images into the box, Eg: Top Center/ Bottom Center/ Center Center/...

To change each subcollection item, please choose a collection you want which has subcollection in the frontpage and click any image, then click on Shortcut item at the right center of the screen, choose Edit Custom Field

An ACF tab appears, you can copy an address image and then click on Select file (or Edit file) to paste in a file URL > Choose Select. 

Note: Please click Save at the Top Right of the screen.

Back to the main page, you reload the page and see the image that you just changed.