Although this thread seems to be old now, here are my 2 cents. I have been doing Then and Now pictures for years now. For me the only way it has worked is to create a grid and vertical helper lines connecting remote points with points closer up on the original picture and either print these or take them on your tablet/mobile phone with you. Overlaying on the screen is of little help as the original picture was taken with a different focal length and the scenery was way different. In some cases it is necessary to take a picture only to find out what reference points can still be used and then go back again. With the picture with grid lines you can walk around and you’ll soon see matching points and also see if you have the correct distance.
For aligning pictures of different size you can use popims, in popims you can set markers on on the same points on both pictues and it will then align and rescale the picture. http://logicnet.dk/PopimsAnimator/
The free version as offered for download does not allow rotating and keeping an uncropped version. However the owner of the Web Site can provide a version that allows rescaling and rotating.
[Here is my colleciton ofThen and Now Pictures on Flickr]
(Re-Photography | Flickr)
Here is an example from an old Postcard. The yellow and orange lines are just a 6x6 grid and the green lines and circles have been added manually.
Firefox is scaring me when I try your app’s link.