If you upload a picture to Lemmy, it's going to get saved by a shit of federated instances.
That's how federation works, but once it happens, it's hard to get all of them to delete it.
The fix is easy:
Upload somewhere else (theres a bunch of images hosts) then make your post point to that image host. Federated instances just have to host the link, so it's good for them too.
I'd love to see something like the RES feature where Lemmy can still show an expandable thumbnail for non-hosted images. RES pulled it off fine years ago, not sure how hard it would be.