MetaMask browser extension
While you can add NFTs as custom tokens in the browser extension, you won't be able to see the artwork image.
MetaMask Mobile App
MetaMask Mobile displays NFTs under the NFT tab. Please remember that some NFTs will not show up on the mobile app, or may not show up correctly.
We pull our NFT data from IPFS and opensea.io, our 3rd party repository for all NFTs.
If your NFT doesn't show up automatically, try the following to make it appear:
Find the NFT's address. On OpenSea, for example, on the NFT's page under Details, there is a blue hyperlinked value labelled "Contract Address". If you click on this, it will take you to the contract's address on Etherscan; at the top-left of that page, there should be an icon labelled "Contract", and to the right, a long string of letters and numbers. This is the address of the contract that created your NFT. Click on the "copy" icon to the right of the address, and you'll have it on your clipboard.
In MetaMask Mobile, tap on the 'NFTs' tab, scroll down and tap on the "+ ADD NFTs" link. Paste the NFT's address from your clipboard into the "Address" box.
Find the NFT's ID. A collectible's ID is a unique identifier since no two NFTs are alike. Again, on OpenSea this number is under "Details". Make a note of it, or copy it onto your clipboard.
Paste the ID into the box marked "ID" in MetaMask Mobile, tap on the "ADD" button, and your NFTs should appear under the NFTs tab.