She has already testified under oath that she took the photos. If she now were to tell the "truth" and admit she did not take them, she would have even more questions to answer: e.g. how she got the photos (and how it was that de Mohrenschildt got a copy signed by Oswald and with her handwriting on it "Hunter of Fascists") why they were faked, how they were faked, and why she lied. Her "immunity" would be only if she told the truth this time (i.e immunity from past perjury).
Actually, Marina has always denied and it was determined conclusively that Marina did not write "hunter of fascists" on the back of the De Mohrenschildt photo, which of course opens up a whole new line of enquiry.
De Mohrenschildt believed that Michael Paine had placed the box with records, containing, the photo in his storage unit, which by itself is somewhat strange since the Paine's have always stated that they did not know the De Mohrenschildt's very well. So, if Michael Paine did indeed place that photo in the storage unit and it wasn't Marina who wrote that Russian tekst, then who did? Who else is involved in the BY photos saga?