Hi,
How to add reciver and sender different icons ?

DEFAULT

@Web
Do you mean like default icons?

Yes

@Web
In file:
oc-content/plugins/instant_messenger/user/messages.php

Above line:
$img = im_get_user_image($u_id);
add code:
          if($logged_is_owner) {
            $def_img = osc_base_url() . 'oc-content/plugins/instant_messenger/img/profile-default.png';
            $u_id = osc_logged_user_id();
          } else {
            $def_img = osc_base_url() . 'oc-content/plugins/instant_messenger/img/profile-default-alt.png';
            $u_id = ($thread['i_from_user_id'] == osc_logged_user_id() ? $thread['i_to_user_id'] : $thread['i_from_user_id']);
          }

Save, then into folder:
oc-content/plugins/instant_messenger/img/

add alt image with name:
profile-default-alt.png