function add_cp_tweaks_css()
{
    $css .= file_get_contents(PATH_THIRD."my_cp_extension/css/cp.css");
    return ee()->extensions->last_call . "n" . $css;
}

function add_cp_tweaks_js()
{
    $js .= file_get_contents(PATH_THIRD."my_cp_extension/js/cp.js");;
    return ee()->extensions->last_call . "n" . $js;
}

// cp.css
.my-message {
    font-weight: bold;
}

// cp.js
$(".t-0").prepend('<div class="my-message">You are not allowed to add pictures. Contact an admin.</div>');

$user_group_to_show_message = 10;

$referer = (array_key_exists('HTTP_REFERER',$_SERVER) ? $_SERVER['HTTP_REFERER'] : '');

// If we're on the Edit page, get Entry ID from referrer and then use that to get the current page URL
preg_match('/.*publish/edit/entry/(d+).*/', $referer, $entry);

// If we're on the Create page, get Channel ID from referrer and then use that to determine if this is a listing entry
preg_match('/.*publish/create/(d+).*/', $referer, $channel);

if (array_key_exists(1,$entry)
    && is_numeric($entry[1])
    && ee()->session->userdata('group_id') === $user_group_to_show_message) {
    // We are on a Edit page, with the right user group to warn, go ahead and add CSS/JS
}

if (array_key_exists(1,$channel)
    && is_numeric($channel[1])
    && ee()->session->userdata('group_id') === $user_group_to_show_message) {
    // We are on a Create page, with the right user group to warn, go ahead and add CSS/JS
}