She voiced outrage on Thursday at a court order demanding she undergo psychiatric evaluation in the case.
The order calls for the tests to be carried out "as soon as possible" to establish whether she "is capable of understanding remarks and answering questions".
Experts have said such tests are normal for these types of investigations. "This regime is really starting to worry me".
Le Pen, who has said the case against her is a violation of her freedom of expression, claims the government of France is employing the tactics of defamation that totalitarian regimes use to discredit political opponents in an attempt "to make them look insane".
Jacky Coulon national secretary of the magistrates union told France Info radio the procedure was "automatic" for charges related to publishing certain kinds of images.
The 50-year-old politician has been charged with 'violent messages that incite terrorism or pornography or seriously harm human dignity, and that can be viewed by minors'.
Among the list of concerns were whether Le Pen is "in a unsafe state from the standpoint of psychiatry or forensic science".
Ms Le Pen added: 'Of course I will not go to this psychiatric assessment and I will wait to see how the judge intends to force me'.
Italy's Interior Minister Matteo Salvini said in a statement: "A court orders a psychiatric assessment for Marine Le Pen".
Le Pen was placed under formal investigation in March on suspicion of disseminating violent images.
Le Pen had shared the images in response to a French journalist who drew a comparison between IS and her party. "Words fail me! Solidarity with her and with the French who love freedom!" he wrote.
But it doesn't look like Le Pen will be making an appointment with any psychiatrists.
Le Pen later deleted the photo at the Foley family's request.
The crime carries a prison term of up to three years in prison and a fine of 75,000 euros ($91,000), according to the report.
One of the pictures showed the body of James Foley, an American journalist beheaded by the Sunni extremists.