I think the only easy way is to override the icon/group view.
You could also filter view_vars for output/img but I am not sure if you can determine with certainty where the view is rendered (you could check owner block context, I believe it is set somewhere).

you where right many thanks

here's the fix 

in owner_block elgg_view_entity_icon i added $vars plus a little css

$header = elgg_view_entity_icon($owner, 'large', $vars = array('img_class' => 'new', 'width' => '','height' => ''));

.elgg-owner-block img.new {
    width: 100%;
    height: auto;
}​

ok so i was wrong of being wrong 

code works but only inside of profile 

but why ?