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Bootstrap 3 has css classes for images -- Responsive images . A preprocess function in template.php can be used to add the css class.
/**
*
*/
function THEMENAME_preprocess_image_style(&$vars) {
$vars['attributes']['class'][] = 'img-thumbnail'; // can be 'img-rounded', 'img-circle', or 'img-thumbnail'
}
And "Clear Cache" (admin/config/development/performance).