Senegalese President Macky Sall attends the Outreach program on the second day of the summit of G7 nations at Schloss Elmau on June 8, 2015 near Garmisch-Partenkirchen, Germany.
A young female journalist, who allegedly first posted the image of the president, was arrested on Sunday, while three others were arrested on Wednesday, said the sources who spoke on condition of anonymity.
The four have not yet commented.
Police investigations are still under way but those involved may be charged with "disseminating obscene images " and "offending the head of state", the police sources said.
The journalist is working with Touba TV station. It is owned by the highly influential Mouride Islamic sect, the second largest religious community in the country
Some people who saw the image said they were shocked by it. But others accused the authorities of attempting to curb freedom of speech.