It's not entirely clear to me when or why an image is locked by the mods, but it seems to indicate a seal of approval at some level. If an image is locked, I think this status should be returned in the api results (locked: true).

Example: for Harry Potter, this logo is the most popular, however it doesn't actually look like the most official logo. Rather, it seems like a logo made by a distribution company in order to organize their catalog or something. However, the actual official logo, here, is not the most popular, however it is locked.

If locked status on an image can be relied on as some sort of seal of approval made by a mod or admin, then I think this status should be reflected in the api, as it allows developers to help ensure that official (or at least mod-approved) images are preferred over merely the most popular images.