How to cache background image (size)

Plone Support Theming
1

If I use background images I get:

The following resources are missing a cache validator. 
Resources that do not specify a cache validator cannot be refreshed efficiently. 
Specify a Last-Modified or ETag header to enable cache validation for the following resources:

http://path/to/image.jpg/@@images/image/large

With 'normal' img tag, the url will not be the same (?), but is there a way to get background images to cache.

I use the following:

  <div class="slide textslide" style="background-image: url(${item_url}/@@images/image/${image_size})">
2

i just meet the same issue, do yo ufind the way yet?

3

may be nignx config set expires 30d;
but don't know if there is any side effect