Allison Coss and Scott Sippola claim the FBI lost or destroyed them during a search of his Upper Peninsula home in December.
Federal Judge R. Allan Edgar said Thursday that's an issue for the trial next week in Marquette and has refused to draft a jury instruction that could cast doubt on the government's case.
Prosecutors deny the FBI lost any photos and insist the pictures don't exist.
Coss and Sippola are accused of demanding $680,000 from Stamos or they would sell photos of him with cocaine and strippers. Coss and the actor met in Florida in 2004.
