Setting JPEG compression for custom image sizes doesn’t work in specific cases

Some suggestions:

If you want to try out the Image Editor API you can try to replace

imagejpeg( $resource, $image, 35 );

with:

$editor = wp_get_image_editor( $image );  
if ( ! is_wp_error( $editor ) )
{
    $editor->set_quality( 35 );
    $editor->save( $image  );
}
unset( $editor );

Also try to test e.g. these parts:

$resource = imagecreatefromjpeg( $image );
if( false !== $resource )
{
    imagejpeg( $resource, $image, 35 );
    imagedestroy( $resource );
}

in a standalone PHP script, to make sure it’s not a PHP or GD related issue.

PS: Here’s a different approach where we try to modify the image quality before the image size is generated.

Testing the plugin

Let’s check the plugin from the main question.

Test install

• WordPress version 4.9-alpha-40917
• PHP 7.0.19

Here’s the generated hello-image (670×300) size, of the SMALL (670 × 377) image, from the question, where we set the quality to 40:

imagejpeg( $resource, $image, 40 );

Here’s the same but with the quality set to 0:

imagejpeg( $resource, $image, 0 );

So it looks like we don’t get the same behavior as described in the question, that when the quality is set to 40, it becomes 0, for the hello-image size of the 670×377 original image.