bviTouch lets blind and visually impaired users explore images on screen by providing musical tones to represent the colors under their finger as they move them about the image.
It is newly available on the Apple App store.
It is free, with no advertising or in-app purchasing.
Support web site with further details is:
http://www.mistisoftware.com/apps/bviTouch/index.htm
App Store link: https://itunes.apple.com/us/app/bviTouch/id1231243570