As another commentator said, the key demonstrates the ownership so you're the owner now. Sometimes that's a bad thing indeed, but blockchains are very unforgiving at their core protocols.

> what if i upload a different NFT smart contract that gives me an NFT of the same image you have?

First off, images have nothing to do with NFTs and vice-versa. Yes, some NFTs links to images but those are uninteresting and hype-driven (or I don't understand it). Once a NFT has been created, you cannot replicate it as it's a single instance of a token (hence the name "non-fungible").

That's what "owning it" means; you have the key.