How to set multi-lingual image paths in a css file for different environments?
The easiest option is probably to use inline styles in your markup for the relevant element(s):

{% set sitePath = craft.config.environmentVariables.sitePath %}

&lt;div class="foo" style="background-image:url('{{ sitePath }}/assets/bg.jpg');"&gt;
    &lt;img src="{{ sitePath }}/assets/logo.png" alt="" /&gt;
&lt;/div&gt;