Custom column into custom taxonomy (img tag with slug-name as file name)

You can use get_term_field() like this:

if ($column_name === 'image') {
    $slug = get_term_field( 'slug', $term_id );
    $columns="<img src="" . $slug . '.jpg">';
}

Or get_term():

if ($column_name === 'image') {
    $term = get_term( $term_id );
    $columns="<img src="" . $term->slug . '.jpg">';
}