about_them_facts on Nostr: To actually put an image (not just a link to the image) on the Bitcoin blockchain ...

To actually put an image (not just a link to the image) on the Bitcoin blockchain would be very expensive. And any other chain, well, maybe it won't be around that long. You can put a hash of the image onto the BTC blockchain, there are plenty of BTC notary services for this exact purpose (either tools/wallets or websites which can do this). A hash would prove the image existed at a certain point in time, and you can use it to prove the image you have hasn't been modified. Not an NFT. I don't have any specific apps for this purpose to suggest.