In a news release posted online in advance of his speech, Mr. Inhofe said that the photographs “were given to me by Lt. Col. Semen Semenchenko, the commander of Donbas Volunteer Assault Battalion and newly elected member of the parliament of Ukraine, during our meeting on 13 Nov. last year” in Washington.

Asked by Rosie Gray of BuzzFeed to explain how the error had come about, Senator Inhofe’s office said that the images were provided during a meeting with a large delegation of Ukrainian commanders and officials who were in Washington last year when he was the ranking member of the Senate Armed Services Committee. A list of the participants in the meeting showed that it included three members of Ukraine’s Parliament and a former Pentagon official, Phillip Karber.

Mr. Karber, who has advocated better equipping Ukraine’s military since early in the conflict, told BuzzFeed that he had only meant to confirm to Mr. Inhofe’s aides that a second set of images, showing the badly mangled corpses of Ukrainian fighters, were authentic.

As questions were being asked about why the senator’s office did not vet the images online itself, Mr. Inhofe — who is perhaps best-known for calling global warming “a hoax” that can be refuted with “biblical evidence” — revealed some apparent confusion about the mechanics of digital photography. He said in a statement that the Ukrainians “gave us these photos in print form, as if” they “came directly from a camera.”