You have to upload the meta data and the image will appear.
Before, you could have followed the tutorial of Network Chucks from Solana.
https://www.youtube.com/watch?v=befUVytFC80
In short, before, you needed to put this information in repository at GITLAB and modify the Solana basic code by forking it, changing it and submitting it for approval. I think it was considered to be too complicated for users so, they changed the procedure.
Since July 2022, if I remember well, the procedure is different. I saw this little on-line tool, that one might use (I did not use it so you would have to try it, but it appears to be from Solana so, it should work). It is for Fungible Token but you can adjust the number of decimals (so, for NFT, put 0 decimal).
https://solana-token-creator.vercel.app/create
I did not upload metadata to IPFS (Inter Planetary File System, I think) since then but I have to do it these days. If you do not get a clear answer from others by the moment that I upload my metadata, I will give you a very clear answer shortly (unless I am stuck, naturally!).