To “pirate” a digital item is to get access to something you’re not supposed to (e.g. software you’re only supposed to have if you buy a license to it). Downloading the image of an NFT is just fine as it’s public content. If you then claim that image is your creation (claim to be the artist) or profit of it (commercial use) that’s more drastic. For many NFTs the graphic attached to them isn’t the valuable part of the asset (e.g. the access it grants, or the voting power it authorizes, or how it interacts with a digital game/space is the key thing that only the owner can do); you having a copy of the thumbnail image doesn’t change the abilities the owner has (and therefore the value of the actual token).