google/guetzli: Perceptual JPEG encoder
Google have released this encoder for JPEGs which promises 20-30% smaller file sizes without any perceptible loss of quality.
JPEGmini - Your Photos on a Diet!
This looks like a really handy tool for reducing the file size of JPEGs without any perceptible loss of quality (in much the same way that ImageOptim works for PNGs)—available as a Mac app or an installable web service.
Related links
WebPonize - webponize.github.io
A Mac app for converting PNGs and JPEGs to WebP.
Retina revolution
You’ve probably seen this already, but it’s really worth bearing in mind: when you’re scaling up JPGs for retina display you can safely reduce the image quality by quite a lot—to the point of getting the exact same file size as a higher quality image that’s half the size.
Essential Image Optimization
Following on from Amber’s introduction, here’s a really in-depth look at image formats, compression and optimisation techniques from Addy.
This is a really nicely put together little web book released under a Creative Commons licence.