By Denny Dyroff, Staff Writer, The Times Transatlantic Sessions A picture of Transatlantic Sessions, a group that is performing May 2 at the Keswick Theater (291 N. Keswick Avenue, Glenside, 215-572-7650, www.keswicktheatre.com), shows a group of musicians walking single file along the edge of what looks like a lake (loch?) in the Celtic region of the U.K. carrying their acoustic instruments. It...