2017 Adding a shadow/gradient to the homepage featured images?

I am using featured images on my 4 Twenty Seventeen homepage panels. The images are about 2000W x 400H which leaves space above and below (I do not want to zoom in with the Cover option etc i.e. I want the full width of the images).

So, to obscure the blank space above and below, I would like to add a shadow/gradient to the top and bottom of Featured Images, which are all centered. However, there appears to be a number of options with transitions and shadows etc

Can someone point me in the right direction?