your right, its pretty difficult in photoshop to scale and position objects, (well i find it difficult)

apparently Piranesi can do it automatically, you can download the trial version online here

http://http://www.piranesi.co.uk/trial/trial_home.asp

i have had a quick play aroung with it but not enough to do anything apart from draw smiliey faces(not one of mine)

the other problem with adding to existing pics is the tone,shadows,colour variation from one pic to the next. this is a little easier to change in photoshop.