Other applications out there in the App Store limit you to backgrounds provided by
the application.
With Spindorax your limit is your photo library. If the picture meets the
requirements, you could use it.
You first select your main background image from your Photo Library and Spindorax will detect
all contours for rectangular shapes. Then you select the contour you want to replace and pick a picture
from Photo Library. The application will apply required perspective to make your composition looks realistic.
Main background and replacement photos can be cropped with Spidorax to fit your desired ratio.
Resulting composition could be saved back into Photo Library or sent by email from inside the application.
Requirements for contour detection:
Image should be sharp
At least one rectangle/square shape on the image, in perspective or not.
Shape border needs good contrast with its surrounding
Online Tutorial 