Fix invalid image helper URLs when using a subdir.

No Issue
- Strip sub-directory from image paths before passing to
  config.createUrl. Since images are stored with the sub-directory
  and createUrl builds a URL with the sub-directory the result
  would be a URL that contains the sub-directory twice.
Merge pull request #4465 from jaswilli/subdir-image-url

Fix invalid image helper URLs when using a subdir.