Image(r) transform and for loop
When using the for loop you don't need to set image as it is set in the for loop. You will also need to define transformedImage on every iteration, so move it within the for loop.

Give the below a go, it should do what you are after.

&lt;div id="header-slider"&gt;
  &lt;ul&gt;
    {% for image in entry.bannerSlider %}
      {% set transformedImage = craft.imager.transformImage(image, { width: 1420, height: 550, position: image.focalPoint,  effects: { grayscale: true, gamma: 1.5}, }) %}
      &lt;li data-transition="" data-masterspeed="500"&gt;
        &lt;img src="{{ transformedImage.url }}"/&gt;
      &lt;/li&gt;
    {% endfor %}
  &lt;/ul&gt;
&lt;/div&gt;