Newbie on Pinegrow

Hi there, I’m a webdesigner based in Milan, Italy.
I’ve done a lot of HTML/CSS for years then I totally converted to Headway, which was really great… until they stopped developing it the last year.
I was considering other "smart themes, but then Pinegrow “grew” on me as a way to develop slimmer and faster themes and give better value to my clients. I think you could agree on this or you wouldn’t be here :wink:

After this short introduction, here’s my problem.

I’ve completed the basic course on Pinegrow Website on creating a WP theme and understood the basics of this tool.
As my first experiment I’m creating a home page with a posts grid, 3 posts on each row, where every post shows, in order, the featured image, the title, the excerpt.
The problem is the featured image.
Since images are all sized differently, I tried to use a div with a background-image property to “cut” all of them at the same size. A pretty simple and common trick, you must agree.
For this I’ve used a “Featured Image bck” action on a div.
Actually, everything works except for the featured image which doesn’t appear. Looking at the generated html code, looks like the div is completely empty.
Looking at the PHP for this element in Pinegrow, the “Featured Image bck” action creates a PHP function with a conditional which, if not satisfied, exits with a “null” and doesn’t generate any code.
I think this is just what’s happening.
But why? Has anyone experienced a similar problem?
Probably it’s something very simple but I really need an help!
Thanks! Cheers