One of the pictures, which she published to protest against comparisons made by a French academic between her National Front (FN) party and Islamic State, shows the body of a man wearing a prisoner-style orange tracksuit with his severed head placed on his chest.
“Those pictures are monstrous,” Prime Minister Manuel Valls said on his Twitter account, calling it “a moral and political error, a lack of respect for the victims”.
Le Pen, who failed to win any regional council in election run-offs last weekend despite a very strong showing in the first round, has strived for years to make her anti-Europe, anti-immigrant party more mainstream.
Seeking to soften the party’s image, she usually avoids the type of provocative acts that her maverick father, FN founder Jean-Marie relishes.
But angered by academic Gilles Kepel saying in a radio interview that although Islamic State and the FN were very different, there were similarities because of their politics of exclusion, she published on her @MLP_officiel account three pictures of dead men with the headline: “This is #Daesh”, using an Arabic term for Islamic State.
One of the pictures showed a prisoner being burnt alive in a cage.
The third one showed a tank driving over another man wearing an orange tracksuit.
Interior Minister Bernard Cazeneuve said the authorities would look into the publication of the images.
However, it could be difficult to launch judicial proceedings as the pictures are not accompanied by any text that glorifies terrorism, which would be against the law.
Le Pen was on Tuesday cleared of inciting hatred for comparing Muslim street prayers to Nazi occupation.
The FN has become increasingly popular since she took it over in 2011.
Opinion polls see her making it to the second round of the 2017 presidential elections, but not winning them.