allCSS.js

// https://developer.mozilla.org/en-US/docs/Web/API/StyleSheetList

var allCSS = 
    [].slice.call(document.styleSheets)
        .reduce(function (prev, styleSheet) {
            try {
                return prev +
                    [].slice.call(styleSheet.cssRules)
                        .reduce(function (prev, cssRule) {
                            return prev + cssRule.cssText;
                        }, ''); } catch(e) { console.error(e); return prev }
        }, '');

allImages.js

var allBackgroundImages = 
  [].slice.call(document.styleSheets)
    .reduce((prev, styleSheet) => {
      try {
        return prev.concat(
          [].slice.call(styleSheet.cssRules)
            .reduce((prev, cssRule) => {
              var img = (((cssRule.style || {}).backgroundImage || '').match(/^url\("(.+)"\)$/) || [])[1];
              if (img) {
                var cssURL = (cssRule.parentStyleSheet || {}).href || document.location.href
                if (!/^(https?|data):\/\//.test(img)) {
                  img = new URL(img, cssURL).href
                }
                prev.push(img);
              }
              return prev;
            }, [])
        )
      } catch (e) {}
      return prev
    }, []); 

var allImages =
  [].slice.call(document.querySelectorAll('img'))
    .reduce((prev, img) => {
      if (img.src) {
        prev.push(img.src)
      }
      return prev
    }, allBackgroundImages)

allImages = allImages.filter((img, i) => { return allImages.indexOf(img) === i })

allImages.join('\n')