If you plan to have a delayed reveal for NFTs minted during your drop, you can add a pre-reveal image. This image will show on the item page for each NFT in your collection until you upload and publish your final asset metadata on-chain.
Click on the Pre-Reveal section and the + icon to upload your image.
Setting, or changing, your pre-reveal image requires an on-chain transaction, which costs gas.
You can also optionally add a description to all of your items.