As alluring as it can be to point folks to `_label` fields, they're intended for developer reference and not use. It can be dangerous to rely on them as they may not represent the exact value (they may have notes, or concatenations of multiple fields).
Often, in places that you might want to use a `_label` field, it's a sign that you're crossing a boundary between two JSON documents, and might want to either (a) rely on the ID or (b) fetch the related document.
A note: in the future the URL you're pulling will actually return a thumbnail, but right now it's pulling back the full-sized image. It's a particularity of our API evolving and supporting older uses where we weren't as explicit as we should have been in hindsight.