@hpthatsme I suggest trying the latest development build: https://github.com/WP2Static/static-html-output-plugin/files/4789479/static-html-output-plugin-6.6.20.zip
You’ll need to deactivate current WP2Static and install this (renamed to Static HTML Output ahead of upcoming release). It solves a lot of issues with the plugin.
I’ve removed settings for offline ZIP creation and recommend people use HTTrack if really wanting an offline site.
Support for relative URL rewriting has also been dropped, as this has potential to cause more trouble than it provides benefits (ie, certain tag attributes must be FQURLs to work) and in general FQURLs are a lot safer.
Please let me know how you go with the new version, it should ensure all background images/URL references from CSS files are properly rewritten.
Cheers,
Leon