There is a difference between being a defendant under arrest for a crime and being a witness to a crime. Rules against search and seizure are intended for defendants. But if one is a witness to a crime, he can be required to testify about it and to refrain from disposing of evidence on it, including cell phone pictures. That being said, it seems that the cops have no authority to require deletion of any photos.
“That being said, it seems that the cops have no authority to require deletion of any photos.”
Seems? I guess you had better pick up a copy of the Bill of Rights and read them over.