You have thie memory limit issue with HTML Rewrite as well!? Wow. Okay, I think your WordPress is running at its very limit right now, you really must change your settings. Did you read this plugin's FAQ? There is a solution in it, please try, I am pretty sure it will work.
You don't have to implement Retina in your theme: you have to implement the "Optimized Responsiveness" in it.
Basically, the Responsive Theme have 3 (more or less) different screen sizes. Depending on each of those supported screen sizes, the layout changes, and the image sizes change as well (again: this has nothing to do with Retina). For example, let's say your posts originally contain a large image that is 1200*800. On an iPhone, the theme layout would change and the image would became 400*300 (or something else). For that, the themes usually just change the size using CSS. If you want to do it right, you should try to have your theme using 400*300 images directly from the start :) You would also have this 400*300 size registered in WordPress as well.
Retina has nothing to do with that, of course. It will do this job, and just load a bigger image in case it is a retina, to fill-up the screen pixel-density.
Is it clear? Maybe not... It is actually a bit difficult to explain, and a very recent issue.
Actually I think those responsive themes are more an annoyance than anything, and if you really want your website to looks great on PC, Tablets, and Phones, you should have 3 completely different themes adapted for each. Much simpler and optimized.