The low-hanging hammock on the left swayed right at the moment the shutter clicked. There are actually two women in the hammock, and there’s a strange blur behind it in the center — probably a child running past. The girl on the grass has tumbled (out of the hammock?), and her legs are up. On the left, the blond girl on the bench has noticed and looks about to laugh.
For a better look, click on the picture — or on “via” above and then on the larger image there.