iOS just uses any <input type='file'> as a button to select an image from camera roll or by taking new picture (at least it does so in a web page, I don't know how it reacts in an application, should be the same).
The filereader plugin will then help you, as it triggers exactly what you want.
I had some problems with the plugin though and had to twick it a little to add possibility of hiding/displaying the <input>.