Spindorax detects rectangular shapes (contours) on pictures and replace them by your desired image, in perspective if required to make it looks like part of the original image.
Other applications out there in iTunes 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
Paid version saves photos at working size (800, 1024 or 1200 pixels)
Free version saves photos at 400 pixels max.