The seventh annual Washington D.C. Photoshow A International comes to the Convention Center Oct. 25-28, featuring equipment displays and sales, photo exhibitions, camera clinics, lectures and films . . . Stewart Copeland, drummer for the Police, is writing a score for the San Francisco Ballet's production of "King Lear." According to London's New Musical Express, he has never seen a ballet or read the play. Copeland did read a synopsis of "Lear," and is using a computer to orchestrate the 20-minute piece for 60 instruments . . . The American Express Co. has pledged $100,000 to the Washington Performing Arts Society's endowment fund campaign . . . The Musician's Committee for Anti-Nuclear Legislation will present a "Washington-Moscow Peace Concert" Saturday at 8 p.m., with cellist Nicholas Anderson and pianist Jeffrey Anderson at the American University's MacDonald Recital Hall. The duo will perform Russian and American music by Shostakovich, Lukas Foss, Samuel Barber and Rachmaninoff. Admission is $5 . . . Shades of King Tut: tickets have gone on sale at 621 Ticketron outlets nationwide for a major art exhibit, "A Day in the Country: Impressionism and the French Landscape," which opens Oct. 23 at the Art Institute of Chicago. The show of 130 French Impressionist landscape paintings, including 42 by Monet, was a hit at the Olympic Arts Festival . . .