BANGKOK, THAILAND — A 25-man US military team conducted searches in Vietnam yesterday for evidence of missing Americans as Hanoi formally agreed to investigate a photograph allegedly showing missing GIs, official Vietnamese media reported.At the same time Vietnamese officials demanded punishment of those involved in producing the photo if it is shown to be a fabrication. Hanoi radio said the 14th joint search for missing US servicemen began Saturday and is expected to last until Aug. 10. It said the search is being conducted in northern and central Vietnam with 25 American experts. The brief radio item did not say whether the investigation would specifically probe the controversial picture made public last week. The fuzzy, black-and-white picture shows three men holding up a sign dated May 1990. Relatives say they are sure the men in the picture are their loved ones missing for more than 20 years.