WASHINGTON, April 28 — A congressman from upstate New York found himself on the defensive Friday over a series of photographs that surfaced showing him at a college fraternity party a week earlier. Democrats seized on the photographs in an attempt to embarrass the congressman, John E. Sweeney, a Republican from the Albany region who attended a party on April 21 at the Alpha Delta Phi house at Union College in Schenectady, N.Y. His visit was initially reported by the Union College student newspaper, the Concordiensis, which said that witnesses described the congressman as being "inquisitive and engaging" and "acting...