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.
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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.