I was downloading images from the uploads folder, then using ImageOptim on my Mac, and reuploading them. I’ve done this before on other WP sites and everything seemed okay.
Ideally I would like retina.js to work, but something has gone wrong with it. It is removing every image on the site, including all of the ones that don’t have a @2x version. Shouldn’t it ignore the ones that don’t have @2x versions? I’m pretty sure it used to work properly, but in the last week or so something has gone wrong and I can’t think what.