Detect blank areas in an image
<p><img src="http://i.stack.imgur.com/iSCpE.png" alt="This is a sample image i use in my app"></p>

<p>I Have an image in which there are some blank areas.What i want is that when the user touches these blank areas a pop up should come asking the user to select an image that can be added to that area.I have done this by just adding a button in the background of the blank area.That worked for a few images.Since the position of the blank area varies for each image that is not a good idea to follow.So is there any provision in ios to detect blank areas and thus make them act like a button. In the image you can see the text "Place your image here".
I want that area to work somewhat like a button.any help</p>
