Select image gallery icon from application menu and select image to use as camera view overlay. Use '+' and '-' icons to zoom image. Touch image icon to toggle original image without transparency, or eye icon to toggle camera view only.
Note that application itself doesn't take photos. Its's only finder. You have to switch to phone camera or use your regular camera to take photo. Postprocessing of photos also has to be done by you.
You can use transparency to make better model image. Try to convert image to grayscale, then apply contrast adjust or edge-finding filter and finally replace white color by alpha. Save as PNG and use in RePrism.
My rephotos of Libeň, Prague.