nostr:npub1n2te0cfhcgmcw23qrf0j7jqwh04umggzgw0tsw4f3em09s50znlszy953z anytime this happens, it is because the server is having an issue with that particular file - the next iteration will be more forgiving.

But you can almost always fix it by running the file through https://tinypng.com and using the result.