Jordy,
I'm adding a comment to this post because I manage a site using the same theme. I've been able to use your plugin with little issue on other sites, but this one is giving me trouble. It also uses the Choices theme the OP mentioned earlier.
I'm using the Client-Side method, and also have debug mode on for now. Seems that the browser(s) isn't requesting the @2x images. The logo image (upper left) is non-retina, and looking at the HTML, it's fetching the non@2x version.
Here's the non retina version:
http://www.theholdinggroup.org/wp-content/uploads/2011/12/holdinggrouphirez-300x147.jpg
Retina version:
http://www.theholdinggroup.org/wp-content/uploads/2011/12/holdinggrouphirez-300x147@2x.jpg
- J