She posted the images following the Paris terror attacks of November 2015, in which 130 people were killed, after a journalist compared her party to ISIS militants.
Later on Thursday, she told reporters she felt "persecuted" and vowed not to attend the psychiatric evaluation.
The president of France's National Rally (formerly Front National) has released an order saying it comes from the magistrates in Nanterre near Paris, calling on her to "undergo a psychiatric examination".
A judge wants Le Pen to take the tests to determine whether she suffers from a mental illness or is "capable of understanding remarks and answering questions", according to The Local France. For having condemned Daesh horrors in tweets, the "justice system" is putting me through psychiatric tests!More news: Devil May Cry 5’s Mega Man Collaboration Is Weird And Wonderful
Le Pen was placed under formal investigation in March on suspicion of disseminating violent images.
Meanwhile, Italy's Interior Minister Matteo Salvini offered Le Pen his support.
As leader of what was then the National Front, Ms Le Pen was comfortably beaten by Emmanuel Macron in the second round of presidential elections a year ago. "She is a political personality but this is not a political decision".
The images showed the bodies of people who were executed by the Islamic State and included the decapitated body of USA reporter James Foley.
The images featured the body of beheaded American journalist James Foley; a man in an orange jumpsuit being run over by a tank; and a captured Jordanian pilot being burned alive in a cage.More news: Trump gets firsthand look at Hurricane Florence distress
Le Pen later deleted the photo at the Foley family's request.
If convicted, she could face up to three years in prison or a $88,233 fine.
Le Pen and her National Rally party, formally known as the National Front, have been targeted by a series of judicial probes, including over the alleged misuse of European Union funds.
Another National Rally lawmaker, lawyer Gilbert Collard, has also been charged for tweeting pictures of IS atrocities.
Le Pen has denounced the case against her, which falls under a law punishing violent images that can be viewed by a minor, as a violation of her freedom of expression.More news: Meghan Markle To Release Cookbook In Support Of Fire Victims
Last week, she repeated warnings that her party's existence was at stake if it didn't manage to win back two million euros in subsidies seized as part of an inquiry into claims it misused millions worth of European Parliament funds.