Image subfolders in HTML5 Boilerplate Build Script

I'm trying to get the HTML5 Boilerplate build script to minify my img folder. It works fine 开发者_如何学运维but it will only compress images within the parent folder.

For example If I had

• img folder
  • img1
  • img2
  • subfolder
    • img3
    • img4
  • subfolder2
    • img5
    • img6

Only img1 and img2 will be compressed.

I realise I can specify a subfolder path, but what I want is for the build script to run through all subfolders.

I've tried modifying the dir.images of the default.properties with img/* and img* but that flags up errors.