Pinterest renders profile pictures as small circular crops throughout its interface — beside pins, on boards, at the top of a profile. Unlike most platforms it offers no tap-to-expand and no save option anywhere, so the image you can see is roughly seventy-five pixels across in most contexts and a few hundred at best on the profile page itself.
Behind that, Pinterest keeps a larger rendition, typically around six hundred pixels square. It sits at an ordinary public address on the same content delivery network that serves pin images, and needs no authentication. Retrieving it is simply a matter of requesting the right size rather than the one the interface decided to display.
It is worth setting expectations on that ceiling. Six hundred pixels is a perfectly good avatar and a poor source image for anything else — it will not enlarge well, and it is not a photograph you can crop into. Pinterest resizes on upload, so whatever the account holder originally submitted is gone regardless of what any tool claims.
One thing people ask about: no, this does not notify anyone. Pinterest has never had profile-view notifications, and fetching a public image creates no signal of any kind — not through this tool, and not through opening the profile normally either. The other common surprise is the generated avatar: accounts that never uploaded a picture get an automatic coloured image built from their initial, and fetching that returns exactly that rather than an error, which looks like a failure unless the tool tells you.