Accessing Global Set Variable
Image fields are an array, because you can have multiple images within that field. Usually you would loop through the array to output your images:

{% if Logo.logo | length %}
    {% for image in Logo.logo %}
        {{ image.url }}
    {% endfor %}
{% endif %}


Or, if you only have a single image in the array, you can just do this:

{% set image = Logo.logo.first() %}

{% if image %}
    {{ image.url }}
{% endif %}


More info on Asset templating here