have this issue too.
the image URL when you see it in browser 'inspect' utility shows
\/images/something/something/image.jpg.
This only happens with SP PB pages not Joomla articles.
Only happened (that I noticed) when update to SP PB Pro 5.0.2
I applied to the fix file site.php as above, but I'm still missing a background image on a banner row. (even duplicating teh row and re-doing the work.
The row has a backgrond image, an overlay with 61% transparency and a logo image on top of this. The logo shows but the background image does not.
Will run a few more tests.