Thanks for your response!
Sorry, I should have clarified — I'm not uploading the images through the WordPress interface. I'm simply placing the image files and their @2x equivalents in the "static" folder (using FTP), and the WP Retina 2x plugin automatically detects and serves the appropriate image. This works fine for my other WordPress sites, even the ones using the WP Retina 2x plugin.
The "special" thing about this particular setup is that I have one WP site nested within another. For example, the main site is at http://www.pretendco.com and the subsite is at http://www.pretendco.com/subsite.
The @2x (retina quality) images work fine on the main site, where they are located at http://firstname.lastname@example.org. They don't work on the subsite, where they are located at http://email@example.com.
Both the main site and the subsite images are using standard absolute paths:
<img src="/static/image.jpg"> and
And the regular non-retina images are displaying correctly on both the main site and the subsite. However, the retina images on the subsite are not being displayed, because the WP Retina 2x plugin is looking for them in the wrong place: /static/* instead of /subsite/static/*.
I'm reasonably savvy but not a full time dev. It feels like a bug, but I'm not sure how to go about isolating it further. Any pointers?