The trick is to use the single parameter context sensitive IMAGE function:
I can see your point but they are two different things, the Retina image is the correct thing to return - the problem is not with the Retina image, it is with the Discourse forum not displaying it with the correct size.
Better would be for the resulting img code to half to size so that it displays as a retina image, and ideal would be for the forum to figure that out for itself.
And indeed, in the code that posts to the forum is the command:
// probably really want the non-retina size here
I expect the forum software is doing some sort of size limitation on it. But I don't know, you'd have to duplicate the issue on the meta.discourse.org forum and then ask there.