How to cache background image (size)

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:


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})">

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

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