If you are looking for something with all the power of photoshop or even photoshop elements that will run on the ipad you aren't likely to find it and probably not free. That kind of app would take a lot of horsepower that just isn't there on the ipad and it would be so complex that its unlikely that anyone after doing that much work would just give it away.
You may have to settle for a combination of apps. It would be easier to sort that out if we knew exactly what functions you are looking for when you say 'nip and tuck'