My CSS file is in :
Project/Web/Support/Styles/file.css
My image is in :
Project/Web/images/image.png
I want this image in my CSS file.
There are two basic ways:
url(../../images/image.png)
or
url(/Web/images/image.png)
I prefer the latter, as it's easier to work with and works from all locations in the site (so useful for inline image paths too).
Mind you, I wouldn't do so much deep nesting of folders. It seems unnecessary and makes life a bit difficult, as you've found.