Let's say I find a really cool design pinned on pinterest and I think it would make a really cool tattoo for TS4, so I go ahead and make it a tattoo. Am I allowed to share that? Would I need to link to that persons etsy or whatever where the picture is actually located or what's the ethics protocol for this kind of situation?

Ethically, it's always best to ask the actual author. The next best option, if you can't get ahold of them, would be credit and link to the original.

Also if your sources are from illustrators, photographers etc (original creations that is), I guess most would have their conditions posted anyway? Noncommercial use is often allowed (with credit), as far as I know; when they have some sort of TOU posted you don't need to ask each one individually as long as you follow their rules.