The rule says two items, not two pictures. The items can be anything that you want, as long as they are not offensive or against policy. You can have more things in your locked storage, but only two items can be displayed in your room at any time. Friends of my son had small stuffed animals, statues, pictures frames, etc. You can have more pictures on the bulletin board on your desk also, as long as they are all pinned up in the same way. (This may vary by squad, some are more strict than others). My son actually took nothing. He literally went with the clothes on his back, his razor, his glasses, and paperwork.
My wife and I took turns writing to him every day, and we also enlisted the help of friends and family to write also. At his HS graduation party we handed out packs of 10 envelopes pre-printed with his address, and asked everyone to write to him at least 10 times. He said that the daily letters were better than any momento he would have had in his room.
At Parents Weekend we brought him a digital picture frame loaded with pics of his friends and family, and he thought that was great.