Thanks Mary,
I've looked into this in more detail and Essential used to employ a form of the grid percentage widths (Shoehorn uses a similar technique) to set the containment of the image box. I seems that it did this without the 'img' tag but in M3.2 I cannot get it to work without (and the 'img-responsive' class).
However as things improved in Essential and to be more progressive with the social icons and the navigation bar underneath the header area - and to be good in all resolutions, the logo had to be come a fixed property entity. This allows the height of the page header to be static and then makes the control of the navbar when scrolling easier. It might be possible to be dynamic, will have to see. But with the percentage width option then this makes large logo's problematic (say using 'span2') unless there is a setting, but too large a span ('span10' for instance) then the social icons get squashed. Catch-22.
Therefore responsive logo's are a matter of give and take in combination with the other functionality around them.
Afterburner being a simpler theme could employ the 'span2' container width technique though and be more responsive than it already is.