To reproduce this, start from "emacs -Q -f eww" and then go to fsf.org. 
Move point over one of the larger images on the page, like the ones that 
say "Featured" above them, and press Z ('shr-zoom-image'). The result is 
that the image is duplicated, and the first slice gets pushed up onto 
the previous line.

Attached is a patch to fix this, with regression tests.