Someone mentioned this on FJ and the poster said that It’s not photoshopped. She found it on Tumblr and that the fan who spotted Derick and Jill at the gym took a picture (I think probably without Jill and Derick knowing). She cropped and lightened the pic so that she could get a better look at Jill's pants,
eta: This is the original pic:
http://dillardfamily.tumblr.com/image/107771559131