Transparent PNG photo shape creator on iPad
This App will allow you to make parts of a photo transparent then save to your camera roll. You will have to draw the shape or use the simple tools to create the mask.  Works well for me as I usually want to remove the background. Not sure of the file format but they are transparent and are great to add to things like Keynote. 
https://itunes.apple.com/nz/app/background-eraser-for-ipad/id532187687?mt=8