Advanced Image: Responsive sizing option

Please add relative/responsive sizing option(s) to the Advanced Image block. At the moment, I can set a Max Image Width for desktop/tablet/mobile, but only in absolute pixels. Adding a % option for that Max Image Width would allow the image to adjust to its parent container for optimal display, without having to guess and compromise for each RWD breakpoint. (An em/rem wouldn't hurt, either, though it wouldn't be as useful as % widths here.) Thanks.