You may want to look at Tools for module developers. Although no iOS tools are listed, it could be that some of the other tools could be an inspiration.
BTW, have you seen XGimp? for iOS. There’s also this for Android.
If you do find some good tools for iOS - or any other platform for that matter - do consider to add them to the Tools for module developers page so that others may leverage your experience and insights. Just edit the main post so that things are easier to find (rather than digging through a large number of comments).
Yours,
Christian