Plugin base URL

I think you are looking for plugin_dir_url:

$url = plugin_dir_url(__FILE__);

$imageurl = $url.'images/someimage.png';

EDIT: Sorry I misread the question… that is only an answer to the linked question. You could check the parent directory recursively until you find the right one:

function base_plugin_dir($dirpath) {
    if (substr(dirname($dir),-7) == 'plugins') {return $dirpath;}
    else {$dirpath = base_plugin_dir($dirpath);}
    return $dirpath;
}

function base_plugin_dir_url($filepath) {
    $baseplugindir = base_plugin_dir(dirname($filepath));
    $url = plugin_dir_url(trailingslashit($baseplugindir));
    return $url;
}

Then you can get the URL using that function from whatever file:

$basepluginurl = base_plugin_dir_url(__FILE__);
$imageurl = $basepluginurl.'/images/wordpress.png';

OR

Better yet just set a constant from your main plugin file to use it later:

$url = plugin_dir_url(__FILE__);
define('MY_UNIQUE_PLUGIN_URL',$url);

So as to use in say /plugin/my-plugin/class/class1.php

$imageurl = MY_UNIQUE_PLUGIN_URL.'/images/wordpress.png';