When you create the field type “image” you have to choose the return value: Image URL
Why is that? Because if the default value is set to “Image Object”, that return an array of values that contains a bunch of info about the image, like: url, width, height, size etc.. but when you set to Image URL then will return the full path of your image: e.g. http://mysite.com/wp-content/uploads/2014/08/myimage.jpg.
If you want more information about the image field type, you can check through here: http://www.advancedcustomfields.com/resources/image/
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