PHP - Cooperative Multitasking, Promises, And Future Ticks

I was trying to prevent bottlenecks when sending emails, and the below post was the solution.

Similar predicament now, but I also need to generate some images first so they may be attached to the emails and the images are generated by a fairly slow command line program that emulates a browser.  Multiple emails might or might need the same image so I plan on creating all the images before queuing the emails.

I believe the futureTick approach executes each tick in series as blocking, but doesn't attempt to execute all of them at once so it doesn't impact the rest of the application.  Promises, however, appear to be more about a structured process for managing callback functions.  As such, the futureTick approach would likely be best for tasks requiring much PHP processing, and promises are more suitable for tasks which PHP is just waiting for a response from some other service.  True?

Would one ever want to attempt to use both approaches?  Seems like it might make sense if the code within a promise was PHP intensive to break it down into multiple futureTick tasks, but not not feed promises into the futureTick as promises are "kind of" non-blocking, true?

To execute the CLI image program, considering https://github.com/clue/reactphp-shell.  Or do you think plan old exec() would be applicable?

For using promises, would it just look something like the following?

$deferred = new React\Promise\Deferred();

$deferred->promise()
->then(function ($rs) {
    // create the first image
})->then(function ($rs) {
    // create the second image
})->then(function ($rs) {
    // create the last image
})->then(function ($rs) {
    // send the first email
})->then(function ($rs) {
    // send the second email
})->then(function ($rs) {
    // send the last email
});

$deferred->resolve($nothing?); 

Or maybe something like the following?  I would be amazed if it works as is, but hopefully is on the right track.

$deferred = new React\Promise\Deferred();
$deferred->promise()
->then(function ($rs) {
    // Create images
    // See https://github.com/clue/reactphp-shell
    $launcher = new ProcessLauncher($loop);
    $shell = $launcher->createDeferredShell('bash');

    $shell
    ->execute('create the first image')
    ->execute('create the second image')
    ->execute('create the lst image')
    
    ->then(function ($rs) use($loop) {
        // Send emails
        // Just showing this way but will actually use kicken's EmailQueue solution
        $loop->futureTick(function () {
            // send the first email
        });
        $loop->futureTick(function () {
            // send the second email
        });
        $loop->futureTick(function () {
            // send the last email
        });
    });
});

$deferred->resolve($someQueueOfImages?);