Using relative image in vendor css and elixir

I've started with Elixir primarily for the benefit of merging my many vendor styles(bootstrap, datatables, semantic, jquery-ui, etc.) I've done that (mostly) successfully. The issue is that this new file public/css/mixed-styles.css uses many relative locations for images/fonts/etc. that assume .css is in node_modules/{vendor_name}/style.cssor similar. My initial desire is to get Elixir to copy all images for all used node_modules into public/images/* or something similar but that doesnt seem possible or feasible.

The alternative that I tried first was to use sass to include these base .css files, but it does not seem capable of including plain .css. I cannot simply rename since these are vendor files.

Any suggestions? I could be barking up the wrong tree here entirely, so another valuable question to have answered would be: How do you manage vendor images and icons using Elixir?