Responsive Images done correctly.
It's simple to throw a photo on a page and call it a day, but doing justice to users on all different browsers and devices is tedious and tricky. Tedious, tricky things should be automated.
Jekyll Picture Tag automatically builds cropped, resized, and reformatted images, builds several kinds of markup, offers extensive configuration while requiring none, and solves both the art direction and resolution switching problems with a little YAML configuration and a simple template tag.
Performance: The fastest sites are static sites, but if you plonk a 2mb picture of your dog at the top of a blog post you throw it all away. Responsive images allow you to keep your site fast, without compromising image quality.
Design: Your desktop image may not work well on mobile, regardless of its resolution. We often want to do more than just resize images for different screen sizes, we want to crop them or use a different image entirely.
Developer Sanity: If you want to serve multiple images in multiple formats and resolutions, you have a litany of markup to write and a big pile of images to generate and organize. Jekyll Picture Tag is your responsive images minion - give it simple instructions and it'll handle the rest.
Writing code is only part of the job; often the harder part is knowing what needs to be changed. Any and all feedback is greatly appreciated, especially in regards to documentation. What are your pain points? See the contributing guidelines, or the issues page for more.