I agree with Pete. This sounds like you are accessing type 'Pageimages' rather than type 'Pageimage' (singular). Since you are bootstrapping ProcessWire, output formatting is going to be off. One of the side effects of that is that all image fields are treated the same (as arrays of images) rather than single images. Since you are using ProcessWire's data for output in this case, I suggest adding a single call to this before you retrieve any pages from PW:
wire('pages')->setOutputFormatting(true);
That will turn it on for all anything returned from the $pages API var.
Or you can do it for just one page, like your banner $page:
$banner->setOutputFormatting(true);
Turning outputFormatting on will ensure you get the correct behavior for output. And if your image field is set to hold max 1 image, then it'll also ensure you only get that when accessing $banner->pastilla_imagen.