gimme-yo-img-src-jamie.js

// no conflict-safe mode (passing $ as an alias into our inner closure)
( function ( $ ) {

	function acfFieldToGroupBg() {

		// cache all our instances of the grab-this element which will be what we work with
		var $jsGrabThis = $( '.js-grab-this' );


		// none of them? why keep going. ABORT!
		if ( $jsGrabThis.length < 1 ) {
			return;
		}

		// it's going to go through all the .js-grab-this that it could find, passing two variables:
		// index, basically, of THIS ONE within our array of .js-grab-this (numbers)
		//					often not that useful, except where you need counters like every nth or "after the first" or ...
		// element : a non-jQuery (vanilla) element / .js-grab-this
		$jsGrabThis.each( function ( index, element ) {

			// element is a vanilla element and not a jQuery selector. let's convert it
			var $element       = $( element ),

				// let's store the value of the image URL (look at the element, and grab its attribute named "src"
				imageUrl       = $element.attr( 'src' ),

				// Might as well find the parents right away

				// starting from this element, go up the DOM until you find an element with a class of .wp-block-group and sto there
				// there's .parents() and .parent()  ... parents keeps going up the DOM nodes until it finds it,
				// 						parent stops at the first parent node... if it doesn't match, you get nothing

				// the main group block
				$blockGroup    = $element.parents( '.wp-block-group' ),

				// try to grab a column parent
				$colParent     = $element.parents( '.wp-block-column' ),

				// was it able to find a column parent?
				// if so $colParent will be an array of (at least) one element
				// you might know count( $some_array ) from PHP, for JS, it's    $some_array.length  (not a function, a property)
				isInsideColumn = $colParent.length > 0,

				// the acf block that we won't actually need in a sec.
				$acfInfoBlock  = $element.parents( '.group-bg' );


			// no image src? not much we can do with this one... NEXT!
			if ( !imageUrl ) {
				// this seems weird, but within $.each(),
				// return is sort of like continue; in a while/for loop
				// return false; is like a break;
				return;
			}


			// this bit of code was added after you told me that you needed to check for cols... so the info is actually more plentiful in the next block
			// the css code is the same in both cases... it's WHAT we apply it to that changes

			if ( isInsideColumn ) {

				// add it to the column's style
				$colParent.css( {
					'background-image' : 'url(' + imageUrl + ')',
				} );

			} else {

				// not inside a column? Group Block it is!


				// let's add that image to our group block....
				// with this function, you can add as many css property : value pairs as you want.
				// In many cases, you don't need to put quotes around the property (the key),
				// 					but because it's JS, you can't have, for instance have a dash. background-image HAS to be quoted...
				// 					but there's a version that it'll accept: backgroundImage.
				// 					tl;dr always quote them and you won't have problems
				$blockGroup.css( {
					'background-image' : 'url(' + imageUrl + ')',
				} );
			}


			// we don't actually need that whole ACF block anymore
			$acfInfoBlock.remove();


			// next in the each loop
		} );


	}

	// do the dew
	acfFieldToGroupBg();

} )( jQuery );