I did the samething but according to my book the answer is 48. 2 pictures from 4 can be chosen in 1/2p(4,2)=6 ways. they change places between themselves and this is 2! and this öakes 12. The other 2 pictures change places between themselves 2!. 12.2! is 24. If we think 4 pictures as 2 seperate groups : 24.2!=48.