Wikipedia:CSPAN

CSPAN is a non-profit organization that has been granted an exclusive government granted monopoly on footage of the United States Congress in order to further US public awareness of the United States Government. Despite its confusing name, "Copyright" is not recognized as a "natural right" within the United States. (Copyright is a privilege not a natural law)

Historic stills from CSPAN footage are thus uniquely suited to the Fair Use clause, and such stills may be used within the English Wikipedia to document historic testimony before the United States Government. (See WP:FAIRUSE)

Legality of Usage (per Public domain)
All video from within the chambers of the United States Congress is exclusively filmed by C-SPAN. C-SPAN's copyright policy states, in part, "Video coverage of the debates originating from the chambers of the U.S. House of Representatives and the U.S. Senate is in the public domain and as such, may be used without restriction or attribution." Therefore this video is in the public domain as a US Government work.

Legality of Usage (per Fair use)

 * 1) Purpose and character:  CSPAN is a non-profit entity, not a commercial entity. Its purpose is to freely share footage of the US Government.
 * 2) Nature of the copied work:  CSPAN footage is of a democratic body engaged in official proceedings.   The application of copyright to public records is heavily contested.
 * 3) Amount and substantiality: CSPAN stills contain only a fraction of information provided by unabridged CSPAN footage and is thus no threat to the original.
 * 4) Effect upon work's value: CSPAN footage is distributed free of charge under an exclusive agreement with the US government.   A still from footage cannot reasonably impact the commercial value of a non-profit organization that distributes its footage free of charge.