Swimmer - One Difference
This is probably one of the trickiest ones in the whole series. There's only one difference, and the two photos look strikingly similar. Here's a hint: don't bother looking at the boy in the back.
Swimmer - Answer
Did you notice her earrings? She isn't wearing any in the first photo, and seems to have a couple in her ears in the second. This was by far the most subtle difference to find.