I recently added a new member to my class list. She signed on and added her information and has been active with messaging other classmates. But she showed up twice on the list with another classmates' yearbook picture. When I went to his profile, I found he was listed as her. So I deleted his profile and then re-added him. Why did this happen? Is it possible someone hacked into my account? Should I change my password?

It sounds like someone joined under another profile for which you may have already added a photo. They would have changed the name of that member during the join process. Your best bet here is to identify who the profile was originally intended for and add a new profile for that person. You will also have to upload a new photo for that profile. Then you can delete the incorrect photo from your new member who joined as well as delete the extra name / unjoined profile as well.