Make sure your helpers.php is added to the files array in composer.json. See this Stackoverflow answer.
For most things, I would recommend creating a class instead.
I have a global-functions.php file, like your helpers.php. But I don't put much into it. It's for functions that I want available globally, especially in views. It's mainly for extremely generic stuff, like str_replace_once() (a version of str_replace() that only replaces the first instance).
Even then, there are arguments for wrapping these sorts of functions in a class. Maybe it would be cleaner for me to have an App\Utilities\String class, and then call String::replaceOnce(). The main benefit to the global functions is that they can be used from any view, without passing an instance of the class to the view as a variable.
In your case, you have a service that handles a specific task. This is better as a class, rather than thrown together with a bunch of unrelated global functions. The location and name of the class is totally up to you. You could call it something like App\Services\ImageManager.