How to add classes to images based on their categories?

I need to be able to: 1.) assign categories to images and 2.) add classes to all images that correspond to their assigned categories.

I was able to add categories to images with the following:

function ug_add_categories_to_attachments() {
    register_taxonomy_for_object_type( 'category', 'attachment' );
}
add_action( 'init' , 'ug_add_categories_to_attachments' );

And I found a function to add a class to images as so:

function add_image_class($class){
    $class .= ' additional-class';
    return $class;
}
add_filter('get_image_tag_class','add_image_class');

But I’m not advanced enough with php to take it much further. I tried this

function ug_add_image_class($class){
    foreach(get_the_category() as $cat) { $class .= ' category-'.$cat->slug; };
    return $class;
}
add_filter('get_image_tag_class','ug_add_image_class');

but no love…

Help? Ideas? Please? and Happy Friday!

Solutions Collecting From Web of "How to add classes to images based on their categories?"

This should hopefully do the trick:

/**
 * Append the image categories to the current image class.
 *
 * @see http://wordpress.stackexchange.com/a/156576/26350
 */

add_filter( 'get_image_tag_class', 
    function( $class, $id, $align, $size )
    {
        foreach( (array) get_the_category( $id ) as $cat ) 
        { 
            $class .= ' category-' . $cat->slug; 
        }
        return $class;
    }
, 10, 4 );

Testing (WP 3.9.1):

If we set the image categories as (for example):

rolling-stones, steel-wheels-tour, wembley

Screenshot:

image categories

and then insert the image into the post, we get the extra classes as expected:

category-rolling-stones category-steel-wheels-tour category-wembley

Screenshot:

html