Uploaded images (images added through media upload box) can be easily wrapped in a div by applying a filter to the image_send_to_editor function, like this:
function wcr_wrap_image($html, $id, $caption, $title, $align, $url, $size, $alt) {
return '' . $html . '';
}
add_filter('image_send_to_editor', 'wcr_wrap_image', 10, 8);
The code above will generate this output: