Why are my photos becoming transparent in book?

I have created several books in iPhoto, but lately, when I place a picture, I can see the background through the picture. I have no way to adjust the picture to make it opaque again. I did not make it transparent to begin with, so I'm not sure why this is happening. I know it is not picture specific because no matter what picture I put in that slot, it still becomes transparent. Has anyone else experienced this?