I wrote a script to do this task for me. If you explain more about exactly what you are starting with, we might be able to adapt it for you.you mean you have a single image file composed of multiple photos? and you now want to cut out the individual photos?
make sure you have the "selection tool" on the toolbar in Preview.
View attachment 666201
open the image, draw a box (selection) around one of your photos then, copy: [CMD][C], new: [CMD][N], save: [CMD]. repeat until done.
I'm on OS X 10.9.5, Preview ver. 7.0. don't know if the tool is any different on later OSs. the big limit is rotate, preview will only let you rotate in 90º increments. if I need more fine detail rotation, I go into Photoshop. if you don't have that, Gimp is free.
not automatic but free.