The images were tweeted with the caption "This is Daesh" (a pejorative Arabic acronym for IS) and showed a decapitated body, a man on fire in a cage, and a victim being driven over by a tank.
Marine Le Pen has over 830,000 Twitter followers.
The prosecutor's office in the western Paris suburb of Nanterre on Wednesday said it had launched a probe into "the dissemination of violent images".
French Prime Minister Manuel Valls responded on his own Twitter account with the words "Madame Le Pen: inflaming public debate, political and moral failing, non-respect for victims".
Interior Minister Bernard Cazeneuve said the photos were "Daesh propaganda and are a disgrace, an abomination and an absolute insult to all victims of ... Daesh".
He alerted the police to look into the tweets "as they do every time these photos are published".
In parliament, ruling Socialist Party MP Bruno Le Roux said Le Pen was "always trying to take advantage of the situation to further her political arguments, which are based on fear and confusion".
The spokesperson for the mainstream right-wing Republicans, Sébastien Huygues, was also critical. "It's the lowest level of politics. It demonstrates that we were right to launch a strong fight against the Front National."
The legal investigation also targets Gilbert Collard, an MP elected with FN backing, who published photos of IS atrocities on Wednesday.
"We wanted to put images to words so that people will stop insulting more than eight million voters," he said in his defence. "Bernard Cazeneuve has become a speech policeman. He would do better to police crime".
Collard was referring to his party's record number of votes won in regional elections on Sunday.
The Front National campaigned heavily over the migrant crisis and terrorism, though the party failed to win control of any regions.
The parents of James Foley, a journalist executed by IS in 2014, expressed outrage at Le Pen's tweets.
"Our family was informed this morning that Marine Le Pen, a French politician, tweeted a shamefully uncensored picture of our son," John and Diane Foley said in a statement.
"We are deeply disturbed by the unsolicited use of Jim for Le Pen's political gain and hope that the picture of our son, along with the two other graphic photographs, are taken down immediately."
Le Pen on Thursday announced she had taken down the picture of Foley.
"I did not know it was a photograph of James Foley," she declared. "It can be accessed by anyone on Google. I learned this morning that his family has asked for it to be removed and of course I took it down immediately."