That double bracket syntax for the column indicates that it is a list of lists. For your example above, you’ll want to first get the first item in that list. That essentially will get you what GetCell would provide. Let’s call that 'app PostImage' Then you can do what I described in my post about getting image URLs from Airtable. That is, to get the image URL you’ll want to get the first element of out of 'app PostImage' and then the ‘url’ property of that object.
-Mark