There is official adobe photoshop "express" but it has mixed reviews
...you're probably better off going through the various "photo editing" apps and downloading any free/trial ones and seeing which you like best, or find easiest to use...then...if you want, buy a "full" version...or just stick with the free one 
I think Gimp (the software I tend to use) did have a mobile version in development at one point but I haven't heard anything of late <_<
