There’s some way to add a wrapper around posted images?

The image_send_to_editor hook passes a whole range of parameters, not just the html. One of them is the size. To get it, hook your filter passing an 8 as the number of supported parameters:

add_filter('image_send_to_editor', 'my_filter_cb'), 10, 8);

and modify your function header to grab them all:

function my_filter_cb ($html, $id, $caption, $title, $align, $url, $size, $alt) {
    ...
}