div.thumbnail-popular
should get you the element you're looking for — unless there is more than one such element, in which case you will need to narrow down your selector.
For example you will need to find out if this particular element belongs to a specific parent, or is the first, second, ... nth child, or any other information about the surrounding elements in the page that you're working with.
The background URL is in a style
attribute on this element, so you will need to extract that attribute as described here. However you will still need to parse the declarations inside the style
value in order to get the URL; I am not sure if it is possible to do this through kimono as I am not familiar with it (I'm not sure what its advanced mode really does, and it's difficult to tell from the lone screenshot that is provided in that help article).