Perfectly solved, @design.danov!
An alternative to write the multiple backgrounds would be to also use the
background shorthand property like you did with the fallback image. With this property all the values for one background are grouped together and you would separate multiple backgrounds with commas:
background: linear-gradient(to bottom, rgba(0,0,0,0.2), rgba(0,0,0,0) 20%, rgba(0,0,0,0) 80%, rgba(0,0,0,0.2)), url(top-image.png) no-repeat left top, url(bottom-image.png) no-repeat left bottom;