Try this:
<pre><?php print_r( get_field('logo_thumbnail', 'product_brand_' . $term->term_id) ); ?> </pre>
This should let you see all of the data within your image field – paste it here if you feel like it.
Also check your field settings where you add the field to a field group in the ACF admin. There are options for returning Image Object, Image URL, and Image ID – which one do you have selected?
