How I do it: touch attachments, then add attachment, from camera or gallery. When choosing from gallery, there is a little circle turning in the top left corner as I scroll through the gallery. When I get to wanted pic, I let the circle stop running, then wait another ten seconds or so, then select the pic.
This isn't foolproof, as the FTF app generally isn't, but it helps me. Have you logged out, then back in recently?