In those valid cases, if you really worry more about the client, than, no matter mobile or desktop, consider this scenario: you always use smaller size pictures (not necessarily the smallest possible, but small enough in size to qualify as slim and still be good looking), that you scale up if necessary. And this would be your low bandwidth version of the site.
When the user explicitly asks for the high bandwidth version, selecting the visible "High Bandwidth Version" link on your site, you use the bigger size pictures, which, hopefully, are not also bigger in size but are better looking too, in comparison.
RWD can't help you with heavy content issues, it's not its job, it's not its purpose, it's not a content related technology, so stop asking that from it.
RWD can only help when it comes to layout. The D in RWD.