I thought it was only for your uploads, not for reposting someone else's note?

Ah, fair. I haven't seen a more general "mirror button" in any other clients besides NoStrudel, only in Blossom-specific management utilities. One way or another, mirroring your own images or someone else's flows the same way in the backend. At the end of the day, it is a REST endpoint that the client calls, and both servers have to be mindful of what each other expects. One missing header on the origin server can be enough for the destination server to reject the media, so you are basically hostage to each dev interpretation of the specs or even of things beyond the specs. Thankfully, there is no such thing in IPFS, as it is basically torrent-style P2P stuff under the hood.