picture mixin for pug.js - http://codepen.io/herrkessler/pen/ZeZKgW

picture.pug

mixin picture(path, name, type)
  picture
    each val in type
      source(srcset=path + "/" + name + "." + val type="image/" + val)
    img(src=path + "/" + name + ".jpg" alt=name)
	
+picture('path/to', 'image', ['jpg','webp'])

// Compiles to
//------------------------
// <picture>
//    <source srcset="path/to/image.jpg" type="image/jpg">
//    <source srcset="path/to/image.webp" type="image/webp">
//    <img src="path/to/image.jpg" alt="image">
//  </picture>