You could print up the pictures on the computer and put them in a plastic toddler photo album book.
Right now we have a few books on how babies are made. When I read them to our daughter, I read sections that say, "When you were in your mother's uterus" as "When you were in your Mama _____'s uterus." We also read her children's books about adoption.
When our son was 2 and we were on the adoption path we opted to NOT tell him of our adoption plans even though he was so verbal he could say things like, "These flowers smell like pollination." Adoption can take years and a child that young has no concept of that amount of time.
We prepared him for adoption mainly by reading him new baby books. They were on our shelf and we read them randomly just as we read any other book. And whenever I could get my hands on a baby I would let him see me holding the baby. We got a call about a just born baby (who I started nursing at 6 hours of age.) When we went to meet her, we told him he was staying with the babysitter while we went and helped someone. I stayed at the hospital and my husband went and got him. He told him I was at the hospital helping a baby. He was still nursing at the time and when I nursed her, he just took it as how one feeds a baby. Actually, I offered to let him nurse at the same time and he said, "Baby first."
We did not want to say sister or adoption or anything like that until TPR was done. That was 3 days for the birthmom, but the birthfather was unknown so his rights weren't terminated until day 70. We did not want to risk saying this was his sister then have her taken away from us. We were afraid he would worry that someone could take him. So, until all TPR was done, we just told him we were taking care of the baby. If he asked why, we just said because she needed someone to take care of her. We finalized about 3 weeks after the birthdad's TPR. When we had our finalization date we told our son that we were going to adopt the baby and she would be his sister and she would stay with us forever. They are now incredibly close and a joy to watch together.