Home prices showed slight improvement in May, according to Standard & Poor’s Case-Shiller home price index, but prices were still down when compared with May 2010. The Washington metropolitan area was the only place in the country where prices went up over the past year, with a 1.3 percent increase. Minneapolis saw the largest decrease in home prices, down 11.7 percent since last May. The report, released Tuesday morning, found that home prices increased for the second month in a row in the report’s 10- and 20-city estimates — up 1.1 percent to 153.64 and 1 percent to 139.87, respectively.