How to get correct bookmark (webloc) icon to show?
Download the correct icon.
Convert it to .icns with CloudConvert.  (Note: If you convert an image to .icns, the image must be 16 × 16, 32 × 32, 48 × 48, 128 × 128, 256 × 256 or 512 × 512)
Right click on the .webloc file and click get info
Drag the .icns file you just made into the icon on the top left corner of get info.

If that doesn't work, let me know.  I'd be happy to help you! :)