I think that padding-bottom is here because I want to specify the minimum height of the element as a percentage of the width of the screen.  If I used % in the height property, it computes it as a percentage of the screen height. So doing it with padding-bottom allows me to specify a height as a percentage of the width.

I don't know why the calculation here does not match the width.  I would guess that it should.

My guess is that if this line was not here, then off-screen thumbnails that have not had their background-image specified yet would be collapsed and scrolling really quickly through a long list of thumbnails would not work right.