How do i add a background image to a imput submit button in my module?
You can attach it to any render array; the submit button itself seems as good as any:

$form['pixelpin_openid_connect_client_pixelpin_login'] = [
  ...
  '#attached' =&gt; [
    'library' =&gt; [
      'pixelpin_openid_connect/pixelpin_logo',
    ],
  ],
  ...
];