Responsive inline images?
You could use Media (7.x-2.x branch) and some modules for managing responsive images (I use Picture).

You should enable this modules and:


add the "media" button for Wysiwyg (I've tried both Tinymce and Ckeditor)
configure media to use the "Picture" module for a custom display mode for files


So, when you add a media to a textarea you can choose the display mode for the file.
For documentation about Picture please read this.

Obviously you could use also other modules, but the logic is the same: associates a responsive style for a display mode for images.