repost-bot can indicate if an image post is a repost of a previous image. It's presently running on sh.itjust.works as @repost@sh.itjust.works.

When summoned to an image post, repost-bot will respond with whether it has seen the image before.

Mod tools are under development. That would allow moderators of communities finer control over its actions. See the issues list for more details.