Once Grav shows any content in the browser, to modify that you need to work with javascript (jquery, etc.). that is, get the object to be changed by name or class and change an attribute …
@JobPhoning hello
I understand that you are using Image ActioncropZoom on your image, and after that you wish to give it an alt= attribute?
If you are doing this in Twig (template file):
{% set header_image = page.media.images|first %} // We're assigning first image in the page folder to a variable
{% set header_image_cropped = header_image.cropZoom(100,300) %} // Using cropping action on our image
{{ header_image_cropped.html('Mona Lisa', 'Photo of a painting', 'image-small') }} // Rendering the actuall <img /> tag
This will give us output of a 100px wide and 300px tall image with html looking like this: <img title="Mona Lisa" alt="Photo of a painting" class="image-small" src="/page/your_image_name.jpg" />
You could as well do it all in one line, just wanted to show you step by step