[NFT] How do I mint 1000 NFTs at once?

Hi! I am a digital artist and I want to publish my work on some NFT marketplace.

I have seen a lot of collections on OpenSea which contain thousands of NFTs. I have got a thousand of arts I want to upload but I don't see how can I do it – OpenSea and Rarible only offer me to upload 1 image at once by hand, so I need to spend a lot of time to upload all my arts one by one.

How can I bulk upload? Is it done through an API? Is there a simple tutorial for this?