Jetpack Photon is a free image acceleration and editing service for WordPress powered sites using the popular Jetpack plugin. Photon reduces the load on the site’s server, providing faster image loading for visitors while processing images without changing the originals or the information in the site’s database. Photon is activated from the Jetpack page in the site’s WordPress dashboard, and enables images to be served dynamically from the global WordPress.com cloud, even on self-hosted WordPress installations.
There are a few limitations, including no cache invalidations, so the images are cached “forever” — requiring a unique name / URL for updated images. Photon is also limited to GIF, JPG and PNG images, though that accounts for most user’s needs. The system only acts on images in posts and pages, as well as featured images/post thumbnails via the image_downsize filter, and will also not upscale images in order to preserve their quality — so be sure to upload images at the maximum required size.
A Photon API allows developers to transform images using GET query arguments.