The imagery could simply be 100% synthetic.
Moreover, the audience would recognize it as such, and would have full reason to think that it's also 100% contrary to the truth. But that wasn't the point of the visual demonstration - the point was to get an emotional reaction from the audience (in the courtroom as well as beyond the fourth wall), and the Klingons got one all right. Too bad they misjudged the Federation psyche and failed to understand that the audience would gasp at the loss of the beautiful Starfleet starship, not the cruel fates of the Klingon attackers...
Really, if we saw such visuals today, they'd most probably be 90% fake, and the 10% real material would only be there because of current technological shortcomings, not because of any desire to stick to the truth.