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).
It seems like you might be describing two different beasts, which could be part of your difficulty:
A codebase that has "dozens and dozens of classes and header files" sounds like a back-end project (written in C or similar), where the end product is an EXE or server app. A codebase where you'd help by updating "placement of a button" is a front-end project (written in HTML or JavaScript), where the output is HTML.
If you've cut your teeth contributing to front-end projects, you'll likely feel more at home contributing to projects where the output is a website. There is a vast difference between working on a project that uses NextJS and contributing to the NextJS engine codebase itself. Finding a project that is using a library you know would be likely much easier to contribute to than contributing to the library itself.