How to add a class to the attachment images

When I’m in the WordPress editor and I add an external image with the ‘img’ button, my html result wil be like this:

<img src="http://example.com/image.jpg" alt="image" />

How can automatically have a class being added to the img tag?
Like this:

<img class="my-img-class" src="http://example.com/image.jpg" alt="image" />

I would like to apply the same principle to the ‘Add Media’ option.
This output is like this:

<a href="mywordpressdomain/image.jpg"><img src="mywordpressdomain/image.jpg" alt="image" width="872" height="867" class="alignnone size-full wp-image-59" /></a>

And I want the output to be like this:

<a href="mywordpressdomain/image.jpg"><img src="mywordpressdomain/image.jpg" alt="image" width="872" height="867" class="alignnone size-full my-img-class" /></a>

I’m new with PHP, so I’m not sure how to this via a filter.
I’m started like this, but I don’t know what to do next:

function image_tag_class($class, $id, $align, $size) {
    return $align;
}
add_filter('get_image_tag_class', 'image_tag_class', 0, 4);

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You could do like this:

function image_tag_class($class) {
    $class .= ' my-custom-class';
    return $class;
}
add_filter('get_image_tag_class', 'image_tag_class' );

in case you wanna know more about actions and hooks.

Difference Between Filter and Action Hooks?
add_filter
add_action